General Motors Corporation (GM) petitions for a writ of mandamus, seeking relief from a magistrate judge’s order directing GM to produce six documents for which GM asserts privilege.1 We grant GM’s petition and remand.
I. BACKGROUND
Beverly Garner was a passenger in a Chevrolet Blazer that was involved in a head-on collision with another vehicle. After the collision, a fire broke out in the engine compartment of the Blazer. Garner died in the accident. Garner’s sons, Kenneth and Steven Baker (the Bakers), then brought a products liability action contending their mother was killed as a result of a faulty fuel pump that caused the engine fire in the Blazer. The Bakers prevailed at trial in 1993. On appeal, we reversed and remanded for a new trial. See Baker v. General Motors Corp., 86 F.3d 811 (8th Cir.1996), rev’d in part by, 522 U.S. 222, 118 S.Ct. 657, 139 L.Ed.2d 580 (1998) and, 138 F.3d 1225 (8th Cir.1998) (ordering remand).
The present discovery dispute centers around a document created by Edward Ivey, a GM employee. The document analyzed the potential expense of the loss of human life per-vehicle due to fuel-fed engine fires. Before the first trial, GM filed a motion arguing that the document should not be admitted because it was irrelevant. As part of its argument, GM asserted that *1053Ivey’s superiors at GM had not requested that he perform the analysis and that Ivey had never distributed the document to other company officials. The district court admitted the document, and Ivey eventually testified to the same facts advanced by GM in its motion.
In 1998, in an unrelated lawsuit involving GM fuel pumps, a Florida trial court ordered GM to produce documents that related to an interview of Ivey conducted by GM’s attorneys. See McGee v. General Motors Corp., No. 92-23582(25) (Fla. 17th Jud. Cir. Feb. 10, 1998). Copies of two of these documents were later posted on the Internet. After reading these documents, the Bakers asked the district court for an order instructing GM to produce a number of documents related to interviews of Ivey conducted by GM’s litigation team.
The district court ordered GM to produce the documents to the Bakers and to the court for review. We upheld the order instructing GM to produce the documents to the court. See In re General Motors Corp., 153 F.3d 714, 716 (8th Cir.1998). However, we reversed the order instructing GM to produce the documents to the Bakers until the district court had conducted a review to determine whether the documents were protected by the work-product doctrine or attorney-client privilege. See id.
After review, a magistrate judge2 ordered GM to provide six documents to the Bakers. Four of the documents are the hand-written notes taken by GM’s attorneys during interviews of Ivey; one document is the hand-written notes taken by a non-attorney member of GM’s litigation team during an interview conducted in the presence of a GM attorney; • and one document is a typed summary prepared by a GM attorney shortly after an interview. The judge rejected GM’s assertion that the documents were protected work product because, in addition to containing the legal impressions of counsel, the documents contained some factual information. The judge also rejected GM’s claim of attorney-client privilege because she found GM waived the privilege by placing the contents of the documents at issue in the case. The judge determined GM had placed the contents at issue by making assertions of fact and using testimony at trial that may be contrary to the information contained in the documents. GM now seeks a writ of mandamus preventing the enforcement of the order.
II. DISCUSSION
When a magistrate judge has rejected a claim of privilege, we will issue a writ of mandamus when the party seeking the writ has no other adequate means to attain the desired relief and the ruling is clearly erroneous. See In re General Motors Corp., 153 F.3d at 715. This extraordinary remedy is appropriate because the judge’s order would otherwise destroy the confidentiality of the - communications at issue. See id.
In its petition, GM continues to assert that the documents in question are protected by work-product doctrine and by attorney-client privilege. In this diversity case, we apply federal law to resolve work product claims and state law to resolve attorney-client privilege claims. See Simon. v. G.D. Searle & Co., 816 F.2d 397 (8th Cir.1987). The parties disagree as to whether Michigan or Missouri law applies to the attorney-client privilege claims.3
*1054We need not resolve this ehoice-of-law dispute because we find the documents are protected by the work-product doctrine and by the attorney-client privilege law of both states.4
A. Work-Product Doctrine
There are two kinds of work product — ordinary work product and opinion work product. Ordinary work product includes raw factual information. See Gundaeker v. Unisys Corp., 151 F.3d 842, 848 n. 4 (8th Cir.1998). Opinion work product includes counsel’s mental impressions, conclusions, opinions or legal theories. See id. at n. 5. Ordinary work product is not discoverable unless the party seeking discovery has a substantial need for the materials and the party cannot obtain the substantial equivalent of the materials by other means. See Fed. R.Civ.P. 26(b)(3). In contrast, opinion work product enjoys almost absolute immunity and can be discovered only in very rare and extraordinary circumstances, such as when the material demonstrates that an attorney engaged in illegal conduct or fraud. See In re Murphy, 560 F.2d 326, 336 (8th Cir.1977). Initially, we note that these documents were prepared in anticipation of litigation and that there are no special circumstances.5
We find the judge clearly erred in holding that the documents were not protected work product. Notes and memoranda of an attorney, or an attorney’s agent, from a witness interview are opinion work product entitled to almost absolute immunity. See In re Grand Jury Proceedings, 473 F.2d 840, 848 (8th Cir.1973) (attorney’s personal recollections, notes and memoranda from interviews are absolutely protected work product); see also Upjohn Co. v. United States, 449 U.S. 383, 399-400, 101 S.Ct. 677, 66 L.Ed.2d 584 (1981) (“[forcing an attorney to disclose notes and memoranda of witnesses’ oral statements is particularly disfavored because it tends to reveal the attorney’s mental processes”). Attorney notes reveal an attorney’s legal conclusions because, when taking notes, an attorney often focuses on those facts that she deems legally significant. In this way, attorney notes are akin to an attorney’s determination as to which documents are important to a case — the latter being something we have also held to be protected work product. See Petersen v. Douglas County Bank & Trust Co., 967 F.2d 1186, 1189 (8th Cir.1992). Thus, we find that the judge clearly erred because she ordered disclosure of opinion work product.
In addition, even if we were to assume the documents were ordinary work product, the Bakers have not shown a substantial need for the documents and that the substantial equivalent of the information cannot be procured by other means. Discovery of a witness statement to an attorney is generally not allowed if that witness is available to the other party. See In re Grand Jury Proceedings, 473 F.2d at 849. A party also does not demonstrate substantial need when it merely seeks corroborative evidence. See Director, Office of Thrift Supervision v. Vinson & Elkins, LLP, 124 F.3d 1304, 1308 (D.C.Cir.1997) (no substantial need when documents sought would merely reinforce known inconsistencies).
Basically, the Bakers seek these documents to search for evidence to counter the expected testimony of Ivey. At the first trial, Ivey testified that the document he created was only a self-assigned intellectual exercise and that the document had *1055not been circulated to other GM employees. But, the Bakers can already counter Ivey’s testimony with the testimony of Ron Elwell. At the first trial, Elwell provided testimony that substantially contravened Ivey’s. Thus, the Bakers have not demonstrated substantial need and undue hardship because they merely seek corroborative evidence to counter the testimony of an available witness.
B. At-issue Waiver of the Attorney-Client Privilege
The judge also found that any attorney-client privilege claims were waived because GM put the documents at issue in the litigation. The Bakers contend that at-issue waiver of attorney-client privilege would necessitate a finding of waiver for any work-product protection. We are skeptical of this argument. See In re Martin Marietta Corp., 856 F.2d 619, 626 (4th Cir.1988) (waiver of attorney-client privilege does not waive protection accorded to opinion work product). However, we need not resolve this issue because we find the Bakers’s argument for at-issue waiver of the attorney-client privilege fails on the merits.
A waiver of the attorney-client privilege may be found where the client places the subject matter of the privileged communication at issue. See Sappington v. Miller, 821 S.W.2d 901, 904 (Mo.Ct.App.1992); McCarthy v. Belcher, 128 Mich.App. 344, 340 N.W.2d 848, 850 (Mich.Ct.App.1983). There are two situations in which at-issue waiver is commonly found. The first is when proof of a party’s legal contention implicates evidence encompassed in the contents of an attorney-client communication — -for example, when a client uses reliance on legal advice as a defense or when a client brings a legal malpractice action. See State v. Campbell, 913 S.W.2d 832, 837 (Mo.Ct.App.1995); see also People v. Mitchell, 454 Mich. 145, 560 N.W.2d 600, 612 n. 27 (Mich.1997) (defendant who asserts ineffective assistance of counsel waives attorney-client privilege). The second is when a client’s testimony refers to a specific privileged document. See, e.g., Charles Woods Television Corp. v. Capital Cities/ABC, Inc., 869 F.2d 1155, 1162 (8th Cir.1989) (applying Missouri law to find no at-issue waiver when witness testified generally about an issue and never mentioned any particular communication); McCarthy, 340 N.W.2d at 850 (attorney-client privilege waived when client testifies on direct examination about a communication).
The Bakers would like to extend the application of at-issue waiver to a situation where a party has used witness testimony and made factual representations that were allegedly contrary to what the privileged documents will reveal. But, courts in both Michigan and Missouri have rejected the extension of at-issue waiver to this type of scenario.6 See State v. Dumas, 898 S.W.2d 689, 690-91 (Mo.Ct.App.1995) (no waiver of attorney-client privilege when defendant made factual allegations in opening statement that were contrary to his attorney’s recollection of events); Kubiak v. Hurr, 143 Mich.App. 465, 372 N.W.2d 341, 345 (Mich.Ct.App.1985) (no waiver of attorney-client privilege when client has merely testified about facts that client also related to her attorney). Thus, we find that the judge clearly erred in her application of at-issue waiver under both Michigan and Missouri law.
III. CONCLUSION
We grant GM’s petition to bar discovery of the documents, and we remand for further proceedings consistent with this opinion.
. GM also seeks appellate review of the order. Prior to oral argument, we consolidated the petition and appeal. Because we grant the petition, we do not decide whether this order was subject to appellate review, and we dismiss the appeal as moot.
. This case was originally before the late Honorable Joseph E. Stevens, Jr., United States District Judge for the Western District of Missouri. After his death, the case was reassigned to a magistrate judge.
. The Bakers argue for Michigan law because they believe that Howe v. Detroit Free Press, Inc., 440 Mich. 203, 487 N.W.2d 374 (Mich.1992), helps their cause. The Bakers would like to import the reasoning used in that case to determine whether the Howe plaintiffs had waived a statutory privilege against the release of probation reports. See id. at 376-77, 383-84. However, the Bakers reliance on Howe is misplaced because that case did not involve the attorney-client privilege and the court specifically noted that not every waiver of privilege claim should be determined using its reasoning. See id. at 383. Moreover, it does not appear that Michigan courts are *1054using the Howe reasoning to analyze waiver of attorney-client privilege. See Franzel v. Kerr Mfg. Co., 234 Mich.App. 600, 600 N.W.2d 66, 74-75 (Mich.Ct.App.1999).
. The Bakers move to strike portions of GM's reply brief relating to the choice-of-law issue. We deny this motion because we find the arguments are a fair response to arguments advanced in the Bakers’s brief.
. The Bakers do not dispute the judge’s finding that the interviews and documents were not for the purpose of furthering a crime or fraud.
. The Bakers’s best authority to support their at-issue waiver argument is In re Southern and Eastern District Asbestos Litigation, 730 F.Supp. 582 (S.D.N.Y.1990). But, that case did not apply either Michigan or Missouri law.