Wis. Stat. § 893.14
Unless otherwise specifically prescribed by law, the period within which a cause of action may be used as a defense or counterclaim is computed from the time of the accrual of the cause of action until the time that the plaintiff commences the action in which the defense or counterclaim is made. A law limiting the time for commencement of an action is tolled by the assertion of the defense or the commencement of the counterclaim until final disposition of the defense or counterclaim. If a period of limitation is tolled under this section and the time remaining after final disposition in which an action may be commenced is less than 30 days, the period within which the action may be commenced is extended to 30 days from the date of final disposition.
Wis. Stat. § 893.14
When an action to recover damages for injuries to the person is commenced as a counterclaim pursuant to this section, the statute of limitations established by s. 893.54 applies. The tolling of the statute of limitation under this section begins on the date the defendant files the counterclaim. The phrase "unless otherwise specifically prescribed by law" applies to counterclaims that were already barred at the time the plaintiff filed his or her claim; such claims are not resurrected by the plaintiff's filing. Donaldson v. West Bend Mutual Insurance Co., 2009 WI App 134, 321 Wis. 2d 244, 773 N.W.2d 470, 08-2289. In determining whether a client exercised reasonable diligence to discover a claim against its attorney, the existence of a fiduciary relationship, rather than excusing a client entirely from its obligation to investigate, is merely one factor to be considered. Under the circumstances of this case, although a fiduciary relationship existed, the client was a sophisticated corporate actor and that its president and CEO harbored suspicions about the attorney's conduct for approximately one year before the transaction in question closed. Those facts gave rise to a duty to investigate, regardless of the fiduciary relationship. Sands v. Menard, 2016 WI App 76, 372 Wis. 2d 126, 887 N.W.2d 94, 12-2377. Affirmed on other grounds. 2017 WI 110, 379 Wis. 2d 1, 904 N.W.2d 789, 12-2377.