"It was fun, especially the way the season is going now for us," Fielder said. "It's always good to have some fun."
The loss dropped the Giants two games behind Colorado for the NL wild-card spot.
Fielder hit his 37th home run, connecting against Merkin Valdez (2-1). The slugging first baseman untucked his shirt before reaching second base and jumped on home plate as his teammates fell over backward in unison in a choreographed celebration.
Fielder's drive gave him career home run No. 151. The triple play, that was No. 1 for him.
San Francisco led 1-0 when it put Randy Winn on second and Ryan Garko on first in the sixth inning. Aaron Rowand followed with a grounder that third baseman Casey McGehee fielded right on the bag.
McGehee touched third and threw to second baseman Felipe Lopez for the force. Lopez's relay to Fielder beat Rowand by a step, and pitcher Braden Looper celebrated by pumping his fist.
"I didn't really notice it until Flip (Lopez) got the ball," said Fielder, who said he had never been part of a triple play at any level of baseball. "I thought, 'Uh oh, it's a triple play."'
It was the Brewers' first triple play since April 16, 1999, against the Cubs and fifth overall.
"I knew we were going to have a chance," McGehee said. "I was just hoping I wouldn't bounce it to Felipe. I knew we were going to have to hurry. I didn't give him the best feed in the world, but he did a good job of turning it."
San Francisco last hit into a triple play on Aug. 5, 1998, when J.T. Snow did it against the Mets.
"I knew what was going on," Rowand said. "He was just playing right there and got a nice candy hop right on the bag."
The game featured several good defensive plays, including diving catches by San Francisco left fielder Eugenio Velez and third baseman Pablo Sandoval, and Milwaukee catcher Jason Kendall.
"It seemed like there were quite a few going back and forth, people making big plays, keeping the game going," McGehee said.
Milwaukee tied it at 1 in the bottom of the sixth when Lopez walked, moved to second on Ryan Braun's single and scored on McGehee's single.
Mitch Stetter (4-1) pitched the 12th to earn the win.
San Francisco starter Jonathan Sanchez had another strong outing, giving up a run on six hits and three walks in six innings while striking out nine. He struck out six of the first nine batters he faced, but received a no-decision when he was removed for a pinch-hitter in the seventh.
In 11 starts dating to his no-hitter on July 10, Sanchez is 4-3 with a 2.75 ERA.
"The pitching gave us a chance," San Francisco manager Bruce Bochy said. "We just needed someone to come through with a big hit."
Looper came into the game giving up a major league-leading 33 home runs, but kept the ball in the park against the Giants. He pitched seven innings, giving up one run and eight hits. It was the first time in nine starts, and only the sixth time in 29 starts that Looper did not allow at least one home run.
Velez doubled down the right-field line to open the game and scored on Sandoval's single to put the Giants ahead.
Giants 2B Freddy Sanchez is expected to rejoin the team Monday. He has been on the DL since Aug. 18 with a left shoulder strain. ... Concerned about the workload this year for RHP Yovani Gallardo, the Brewers said Sunday they would skip his next start and pitch their ace on Sept. 15 at the Chicago Cubs. Gallardo has pitched 175 1-3 innings, up 24 innings from last year when he had a knee injury.Sign up for breaking news alerts from FOXSports.com!