In the 12th, Mike Fiers (0-2) relieved Jim Henderson to start the frame and promptly surrendered McCutchen's blast, which evened the Pirates' four-game series against Milwaukee at one win each.
Vin Mazzaro (2-0) earned the decision after hurling two scoreless innings.
Jose Tabata collected three hits for the Bucs.
Jean Segura notched two RBI, including a solo homer, while Ryan Braun and Norichika Aoki each banged out three hits for the Brewers.
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