Looking to win their fifth straight on a seven-game homestand, the Giants try to hand the Brewers a fourth consecutive defeat and keep Mike Fiers from winning his fifth start in a row Saturday night.
Posey went 5 for 5 with three RBIs as San Francisco (72-62) rolled to a 13-2 win in Friday's series opener. Posey is 20 for 40 with five home runs and 13 RBIs in his last nine games for the wild card-leading Giants, who are 8-3 at AT&T Park since losing 21 of 27 there.
''We are more loose and having fun again,'' said outfielder Gregor Blanco, who has homered in two straight games and has six RBIs in three.
Milwaukee (73-61) holds a 1 1/2-game lead over St. Louis in the NL Central, but it's dropped six of eight and totaled five runs during a three-game losing streak.
Fiers (4-1, 1.54 ERA) has allowed four runs, 10 hits, struck out 32 and walked four in 28 innings over four starts since he was recalled from Triple-A Nashville on Aug. 9 to replace the injured Matt Garza in the rotation.
The right-hander gave up a two-run homer and struck out seven over seven innings of a 4-3 win over Pittsburgh on Sunday.
"It has been incredibly impressive,'' teammate Ryan Braun said. ''I don't think anybody foresaw him coming up and being the best pitcher in baseball the last four starts.''
Fiers, who made four relief appearances in June, went 9-10 with a 3.77 ERA in 22 starts for Milwaukee in 2012, but he was 1-4 with a 7.25 ERA in 11 games - three starts - in 2013.
''I'm just so pumped up to be up here and contribute to this team," said Fiers, who is making a serious case to remain in the rotation when Garza returns next week from a ribcage strain.
Fiers allowed a run and two hits in 1 1-3 innings of relief against San Francisco last season. He has never faced Posey.
The Giants will turn to Jake Peavy (2-4, 3.18), who appears to have finally settled in since coming over from Boston on July 26.
The right-hander posted a 4.82 ERA while dropping his first three starts for the Giants, who scored six runs with him on the mound. He allowed three runs and nine hits in 5 2-3 innings of a 3-1 loss at Milwaukee on Aug. 7 but has since gone 2-1 with a 1.71 ERA and received 17 runs of support in three outings.
Peavy gave up three runs - one earned - in seven innings of a 3-2 home defeat to Colorado on Monday.
Peavy is 3-4 in 11 starts against the Brewers despite posting a 2.73 ERA. He also gave up two runs in six innings while not receiving a decision in a 6-2 loss to Milwaukee at Fenway Park on April 4.
Jonathan Lucroy is 4 for 5 with a home run and two doubles against Peavy this season, and he's batting .400 with four doubles in four games against the Giants in 2014.