The midgame turnaround came so fast that the crowd wasn't done cheering Braun's double when Fielder went deep.
"I don't even know if I heard the ball come off Prince's bat," Brewers manager Ron Roenicke said. "I knew it was a good swing and came off nice, but when you can't hear the ball, the sound of it, because of all the people yelling. I wasn't sure what was going to happen there until I saw the ball flight."
At least for one game, the bitter NL Central rivals avoided any on-field confrontations in their first postseason matchup since the 1982 World Series.
That's despite an already tense atmosphere that gained some steam when Brewers starter Zack Greinke let it slip on Saturday that some of his teammates don't like the Cardinals' Chris Carpenter - a comment that drew a stern rebuke from Cardinals manager Tony La Russa.
Greinke hinted that he heard a few comments from the Cardinals' dugout Sunday, but said it was "no big deal."
The atmosphere was tense even before the first pitch, as La Russa was showered with boos during pregame introductions. He calmly tipped his cap to the crowd. La Russa said afterward that he hoped the tension wouldn't overshadow the competition.
Greinke struggled, but reliever Takashi Saito got Cardinals star Albert Pujols to ground into a key double play in the seventh. Francisco Rodriguez pitched a hitless eighth, and closer John Axford threw a hitless ninth for a save.
Game 2 is at Miller Park on Monday night. Shaun Marcum starts for the Brewers against Edwin Jackson.
David Freese hit a three-run homer off Greinke in the fourth, and the Cardinals led, 5-2, in the fifth.
But Milwaukee made it tough on Cardinals starter Jaime Garcia, who left after giving up Fielder's homer. Garcia, who hit Fielder with a pitch earlier in the game, gave up six runs and six hits in four-plus innings with three walks. He took the loss.
Greinke earned the win despite his uneven outing, giving up six runs and eight hits in six-plus innings. He left the game to a standing ovation after giving up a leadoff single to Rafael Furcal in the seventh.
The Cardinals took a three-run lead into the fifth before Garcia allowed a leadoff single to Corey Hart and a double to Jerry Hairston Jr. Braun hit a two-run, ground-rule double to right, and with the crowd still saluting him, Fielder hit the first pitch from Garcia deep to right for a two-run homer, giving the Brewers the lead.
Fielder then showed off his repertoire of celebrations, giving the team's "Beast Mode" gesture upon his arrival at home plate and exchanging mock knockout blows with Braun as he trotted back to the dugout.
That was it for Garcia, who left with no outs in the fifth and his team down, 6-5. That wasn't the end of trouble for the Cardinals, though.
Reliever Octavio Dotel fielded Rickie Weeks' grounder and threw the ball away, allowing Weeks to go to second on the error.
Betancourt - who batted .252 in the regular season with 13 homers - then sent a 2-1 pitch from Dotel deep to left, where it flew into the Brewers bullpen and was fielded on the fly by Milwaukee bullpen catcher Marcus Hanel. Hanel pumped his fist, Betancourt circled the bases, and the crowd continued its inning-long eruption.