The rookie left-hander has even surprised himself.
Duensing (3-1) scattered eight hits and three walks over seven innings to win his third straight decision. He struck out six.
"I've surprised myself at how well I'm doing," Duensing said. "I always knew I could throw strikes and stuff like that, but the results I'm getting is something I'm not really used to. I'm not a strikeout guy. I usually let the defense play and let the hitters put the ball in play. But lately I'm throwing the ball really well and it's a good surprise."
Mauer drove in two runs and raised his batting average to a major league-best .366. Michael Cuddyer added three hits and two RBIs for the Twins, who avoided getting swept by the last-place A's.
"We needed this pretty desperately and we got it today," Minnesota manager Ron Gardenhire said. "It started with our pitcher."
Oakland started its own rookie left-hander, but Gio Gonzalez struggled again. Gonzalez (5-6) lasted three-plus innings, allowing seven runs and eight hits.
Meanwhile, Duensing has provided a boost for the Twins in a starting role.
He was 4-6 with a 4.66 ERA while starting for Triple-A Rochester, but 14 of his 15 first appearances with Minnesota were out of the bullpen. He has a 5.34 ERA as a reliever.
When fellow lefty Francisco Liriano went on the disabled list, Duensing entered the rotation. He's done what few of the team's young starters have managed to do -- give the Twins quality innings each time out.
He's gone at least five innings in each of his six starts and hasn't allowed more than three runs in any of them. Duensing also might be putting himself into consideration for a spot in the rotation next season.
"We've been talking about a lot of guys in there next year, and when we get to next year, we'll get there," Gardenhire said. "What's impressive is he's stepping in there in a big situation for us when we've got pitchers going down and proving that he can pitch at this major league level and use all his pitches like that. That's impressive, and much-needed."
Oakland tried to apply pressure, but Duensing effectively worked his way out of trouble.
The A's got the leadoff hitter on base in five of Duensing's seven innings, but left 10 runners on. Duensing stranded seven runners in scoring position.
"He had some pretty good stuff, pretty good curveball, some really nice movement on his two-seamer, and he located the ball pretty good," said Oakland's Rajai Davis, who finished with three hits. "Unfortunately, we were unable to capitalize on some opportunities that we had. He made some good pitches under pressure situations."
Minnesota gave Duensing an early advantage.
Cuddyer singled in two runs in the first inning and Mauer scored on a groundout by Delmon Young. Mauer homered to the opposite field against Gonzalez leading off the third, just clearing the left-field wall.
Gonzalez won three straight decisions from July 25 to Aug. 10, giving up four runs in 19 2/3 innings. Since then, he has a 7.55 ERA in six starts.
"Gio just didn't have command of his pitches today," A's manager Bob Geren said. "It seems when he missed he walked guys, and then he'd come in and it would hit the fat part of the plate."
Minnesota RF Jason Kubel was scratched from the lineup before the game with a sore neck. ... Slumping 1B Justin Morneau was rested with a strained lower back, the result of a slide into first base Friday. Morneau had played in 18 straight games, but had just three hits in 39 September at-bats. He is hitting .201 since the All-Star break. ... Davis was caught stealing in the third inning. He had stolen a base in four straight games, which was the fourth-longest streak by an A's player. ... Oakland went without a stolen base after stealing five in Saturday's victory. The A's had 12 steals in their previous four games. ... Oakland has started a rookie pitcher in 77 of its last 95 games.