For the second consecutive start, the Brewers' 15-game winner was knocked around. This time, Peralta lasted just three innings in the shortest outing of his career.
Peralta surrendered six runs on nine hits and Milwaukee lost for the sixth time in eight games, falling to the San Francisco Giants 13-2 on Friday night.
"I fell behind pretty much everybody (and) that's what you're going to get," said Peralta, who walked two and struck out one. "I have to get back to my thing that I was doing early in the season and just work hard. The past two starts have been terrible."
Peralta (15-9) was tagged for seven earned runs in five innings during his previous start before getting roughed up by the Giants.
Buster Posey went 5 for 5 and drove in three runs , Gregor Blanco hit a two-run homer to back Ryan Vogelsong's first victory in four starts and the Giants scored a season high in runs and have their longest winning streak since five straight victories in early June.
"We need to get (Peralta) right, get him back where he's going deep in games and keeping the run total down," Milwaukee manager Ron Roenicke said. "They racked up a lot of hits off of him in not very many innings. That's not a good sign."
Help could be on the way for the Brewers' pitching staff.
Right-hander Matt Garza pitched a simulated game in what likely was his final test before returning from the disabled list to the rotation next week.
As long as Garza comes through the outing without further problems when he reports to AT&T Park on Saturday, he could start as soon as Thursday at home against St. Louis.
Milwaukee scored its only runs on Lyle Overbay's RBI groundout in the second and a sacrifice fly by Carlos Gomez in the fifth.
The Giants' 19 hits were their second most in the 15-year history of AT&T Park behind 20 on Aug. 14, 2001, against the Marlins.
Vogelsong (8-9) snapped an 0-5 skid at home, where he had received zero runs of support in each of his previous five outings at AT&T Park. He gave up two runs and four hits in seven innings.
Hunter Pence extended the NL's longest active hitting streak to 11 games with his first-inning RBI single.
The Giants scored in each of the first five innings at home for the first time since June 13, 2012, when Matt Cain pitched a perfect game.
Giants: C Hector Sanchez hopes to have recovered well enough from two concussions to be able to play a month of winter ball back home in Venezuela, even if he is a designated hitter or 1B. Manager Bruce Bochy is rooting for that scenario to give Sanchez more at-bats considering his time missed this season. He's unlikely to play again in 2014.
Brewers: RHP Mike Fiers (4-1, 1.54), 4-0 with a 1.29 ERA as a starter, makes his fifth start and ninth appearance in the middle game of the series. The Brewers' next victory will match their season total from last year.
Giants: RHP Jake Peavy (2-4, 3.18) faces the Brewers for the third time this season looking for his first win after two outings while with Boston. ... RHP Yusmeiro Petit, who set a major league record by retiring 46 consecutive batters, will remain in the rotation after his win Thursday pitching in place of struggling RHP Tim Lincecum.