John Axford converted his 44th save in 46 chances with a perfect ninth, including the last 41 in a row, but had to pause with two outs in the ninth when the Cubs-Cardinals score was displayed after Alfonso Soriano hit a three-run homer.
Milwaukee has made three previous trips to the playoffs, but never was assured a spot until either the next to last day or final day of the season.
Fielder's homer off Chris Volstad was his 35th this year, but that was the only mistake the lanky right-hander made. Volstad deserved better. He scattered five hits and struck out five, but hasn't won a start since July 10.
The Marlins tied it in the fourth when Sanchez doubled and scored on Petersen's two-out single. Braun made a diving catch in the fifth, setting the stage for his big at-bat.
"That may have been the turning point of the game," Marlins manager Jack McKeon said.
General manager Doug Melvin said his goal coming into the season was to drop confetti from the Miller Park roof, just like in 2008. In the offseason, he decided to keep Fielder, a free agent to be, and built the starting rotation by trading away his farm system's top talent.
In the span of two weeks in December, Melvin dealt his starting shortstop, backup center fielder, top pitching prospect, top hitting prospect and two pitchers who projected to be no worse than major league relievers.
The returns included 2009 AL Cy Young winner Zack Greinke and Shaun Marcum. Melvin said the Brewers were all-in, just liked they were in 2008 when they traded for CC Sabathia at midseason.
The gamble paid off again, but it took longer than expected for rookie manager Ron Roenicke.
Milwaukee remained in third place behind Pittsburgh and St. Louis on July 25. Milwaukee then won 27 of 32 to take a 10½-game lead in the NL Central.
The nation grieved for those hurt, killed and affected by the Boston Marathon bombings. After one of the suspects was caught on Friday — following a day-long lockdown and manhunt — sports returned to Boston over the weekend.
Milwaukee joined the NL in 1998 and watched St. Louis win seven division titles while the Brewers remained mostly in the bottom of the division.
Things began changing when Attanasio, an L.A. investment banker, purchased the Brewers in September 2004 from Commissioner Bud Selig's family and steadily raised the payroll, up to more than $85 million to start this season.
Milwaukee holds losing records against the Phillies, Braves and Diamondbacks — the other three NL teams in line to join them in the postseason. This time, the Brewers come in a team built on pitching and still has a potent offense.
"It's the most fun you can possibly have," Attanasio said. "The thing about all our guys, they really pull together."
NOTES: Marlins closer Leo Nunez admitted Friday to using fake documents and a fake ID to sign a professional contract more than a year ago and Dominican authorities arrested a man accused of falsifying Nunez's documents. ... With Nunez gone, McKeon will finish the season mixing and matching who will close games. ... Morgan received an award as the team's high energy player of the year, and his alter ego, Tony Plush, got a second trophy in a lighthearted moment. ... Greinke (15-6, 3.75 ERA) faces Marlins RHP Alex Sanabia (0-0, 1.29) in the second of this three-game series on Saturday night.
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