Shaun Marcum (9-3) pitched seven solid innings to push the Brewers back into first place in the crowded NL Central standings. Pittsburgh and St. Louis are one game back.
Lucroy's two-run single highlighted a three-run second inning off Matt Cain (8-6) that gave the Brewers a big lift with All-Star Ryan Braun getting the night off to rest his strained left calf.
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Francisco Rodriguez pitched a perfect eighth and John Axford held the Giants scoreless in the ninth for his 27th save.
Milwaukee outfielder Nyjer Morgan spiced things up after making a catch against the wall in left-center field that robbed Nate Scheirholtz of extra bases in the seventh. Morgan exchanged words with fans in the bleachers, pointed at some and crossed his hands in a taunting gesture.
That only riled up the fans at AT&T Park more, in the bleachers and beyond. Morgan continued bobbing his head and pointing at the crowd, and he was booed relentlessly by fans walking into the dugout after the final out.
Not that it did much to spark San Francisco.
Aaron Rowand's solo home run in the sixth and Pablo Sandoval's RBI groundout in the first were all the Giants could squeeze out of Marcum. The right-hander allowed four hits and two runs, striking out five and walking none.
Marcum settled down and retired 11 straight hitters after Andres Torres' leadoff double in the first. Not until Aubrey Huff's two-out single in the fourth did a Giant reach base, and by then Milwaukee was well in control.
The Brewers scored three runs in the second capped by Lucroy's two-run single. Left fielder Cody Ross' throw home on the play beat Betancourt, but catcher Eli Whiteside was playing behind the plate and couldn't corral the ball, allowing Milwaukee to go ahead 3-1.
Betancourt drove in his second run with a two-out single in the sixth that sneaked between the infielders and into left, scoring Rickie Weeks from third to give the Brewers a 4-1 lead.
That was more than enough for a short-handed Milwaukee team.
Braun was given the day off as part of a plan to ease his return from a strained left calf that forced him to sit out more than a week before the All-Star game, which he also missed. He had started the last two games in Arizona and the team arrived late in San Francisco, so manager Ron Roenicke figured it would be an ideal time to rest his slugger. Mark Kotsay was 0 for 4 in Braun's place.
Rowand tried to bring the Giants back with a pinch-hit home run in the sixth. The solo shot was Rowand's fourth career pinch-hit homer and second this season, trimming Milwaukee's lead to 4-2.
Brewers backup OF Carlos Gomez will have surgery Monday to repair his broken left collar bone, Roenicke said. The team is hopeful Gomez will return this season. ... Brewers RHP Brandon Kintzler is scheduled to have a screw inserted in his forearm to help a stress fracture heal. His chances of returning this season are less likely. ... Giants C Buster Posey had the screws removed from his surgically repaired left ankle, manager Bruce Bochy said. Posey is out for the season after tearing three ligaments in his ankle and fracturing a bone in his lower leg in a home-plate collision with Florida's Scott Cousins on May 25. ... The Giants' 48th straight sellout this season pushed the total to 2,005,900.