Come Sunday, the Brewers swapped their barbs for bats -- and just kept bashing.
The Brewers celebrated wildly as the big hits came during a rapid-fire rally.
"It's the playoffs, bro," Fielder said. "You've got to let it all out."
Braun launched a two-run, 463-foot homer in the first inning and added a two-run double during a six-run burst in the fifth. Fielder hit a two-run homer and the typically light-hitting Betancourt added a two-run homer to cap it.
The midgame turnaround came so fast that the crowd wasn't done cheering Braun's big hit 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 he said it was nothing out of the ordinary.
"They're yelling from the dugout some, but most teams do that," Greinke said. "Everyone always makes fun of me grunting when I throw a fastball. It's kind of funny sometimes, but 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 -- although he said he had a sense that some fans and media members would be disappointed if there aren't any repeats of the on-field confrontations the teams have had in the recent past.
"I don't want our players and their players to be egged on, and I don't think they will," La Russa said. "We're going to play as hard and good against each other as we can."
Greinke struggled at times, 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.
"We'll come back out," Cardinals star Lance Berkman said. "The same thing happened to us in the first game against Philly. We were able to regroup."
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. It that wasn't the end of trouble for the Cardinals, though.
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.
Betancourt has taken plenty of criticism this season, but might be among the rare group of athletes who say they don't pay attention to the critics and actually mean it. Through an interpreter, Betancourt said he manages to avoid criticism because he doesn't speak much English.
"I don't really pay attention to what the critics say," Betancourt said.
With the score 8-5, Pujols came to the plate with runners on first and third and no outs in the seventh. Pujols broke his bat on a double-play grounder -- a run scored, but the Brewers had limited the damage.
Betancourt doubled in the seventh and scored on a single by Jonathan Lucroy.
Axford sported a heavily bandaged right forearm in the clubhouse after being hit by Jon Jay's liner on the final play of Sunday's 9-6 victory. X-rays taken afterward were negative.
"Everything is good," he said.
The Brewers and Cardinals split an 18-game series evenly this season, a sign of what has been one of baseball's most intense rivalries in recent years. The Cardinals' success against the Brewers in the final month of the season was one of the main reasons they climbed back into playoff contention.
St. Louis won six of its last seven games against Milwaukee, including a three-game sweep at Miller Park.
The animosity between the two teams spilled into this week, when Greinke told reporters Saturday that some of his teammates don't like Carpenter because of his "phony attitude."
La Russa said he got an umpire's warning after Garcia hit Fielder with a pitch in the first inning, right after Braun's homer. But the Brewers said they didn't think the pitch was intentional, and La Russa said the team's recent history probably affects the umpires' attitudes.
"I certainly can't fault the umpire," La Russa said. "But, you know, you can't go out and argue those things, or you get thrown out. I didn't say anything. What I would have said is, if you watched the way Jaime pitched that whole inning, every fastball he threw was in that same area, out away from the right hander or in on Fielder. They just looked bad, but he was just trying to get the ball somewhere near the glove."
Jackson struggled in his first outing with the Cardinals against the Brewers, giving up 10 runs (eight earned) in seven innings at Milwaukee Aug. 3. But he was better in two subsequent starts against the Brewers, allowing four runs (three earned) in 13 innings. ... Marcum is coming off a rough outing in the Brewers' Game 3 NL division series loss at Arizona, giving up seven runs in 4 2/3 innings.
Information from The Associated Press was used in this report.