Mauer, who has three more spring training games to complete his conversion from a three-time Gold Glove-winning catcher to a full-time first baseman, bobbled a ball hit by Travis Snider with two outs and the bases loaded. The error allowed Tito Polo and Jordy Mercer to score the tying and go-ahead runs.
Oswaldo Arcia hit a solo home run, his second of spring training, for the Twins off Pirates left-hander Tony Watson.
STARTING TIME: Ricky Nolasco, Minnesota's opening day starter, struck out three in four scoreless innings, giving up one walk and one hit in his final outing of the spring.
''He pitched great,'' Twins manager Ron Gardenhire said. ''He threw the ball very, very well. He's feeling really confident, and I'm really confident. He took himself out basically and said he felt good.''
Nolasco started on opening day last season for the Miami Marlins and in 2009 for the Marlins as well.
''It's an honor,'' Nolasco said. ''I'll just go out there like I have my whole career. Everybody's excited to get it going.''
Nolasco said he felt polished enough to end the outing after four innings. He finished spring training with a 5.50 ERA in five starts.
''It's the last start of spring,'' said Nolasco, whose Twins open at division rival Chicago on Monday. ''I was just trying to go four innings. I've been ready for about two weeks now. I've felt good all spring. I'm ready to go.''
Pirates starter Brandon Cumpton, who made his big-league debut for the Pirates last season and wound up starting five games, struck out four and gave up two hits in five shutout innings.
''I'm still working on my delivery and my mechanics,'' said Cumpton, who spent most of last season in Triple-A Indianapolis, where he had a 6-7 record and 3.32 ERA in 19 starts, 20 appearances. ''Overall, it was a pretty decent day for me. It's a big confidence builder for me, too, seeing guys like that in the lineup and getting to throw against them.''
CLOSING TIME: Pirates closer Jason Grilli struck out one in one inning. He has not allowed a run this spring after pitching four innings in the Grapefruit League, plus additional work in minor league games. He said he is scheduled to pitch one more exhibition game against the Phillies.
''Opening day is the best day for baseball,'' Grilli said. ''Everybody is excited. People can see that. It's time to play ball, right? Everything is fine. I'm ready to go. I will have gotten my nine innings in. It's been a great spring training.''
TRAINING ROOM: Twins shortstop Pedro Florimon continued his accelerated spring training, having been limited to seven games following an emergency appendectomy at the end of March. Florimon went 0-for-3 on Wednesday and is hitting .150 (3 for 20).
''I'm going to need a few more at-bats to get ready for the season,'' Florimon said. ''I've just been working hard every day.''