The ace right-hander walked two batters, fanned five and allowed just one runner to get to second base, when Corey Hart doubled in the top of the top of the third.
Bengie Molina led San Francisco with two hits and four RBI. Fred Lewis had three hits and scored twice, and Aaron Rowand had two hits and one RBI.
One run scored on an error, while Pablo Sandoval added two hits for the Giants, who won twice in the season-opening three-game series at AT&T Park.
Milwaukee's lone run scored on Jason Kendall's seventh-inning sacrifice fly.
Losing pitcher Manny Parra (0-1) was tagged for five runs and six hits in 4 1/3 innings.
Thursday's loss ended the Brewers' streak of winning their first series at five straight seasons.
Copyright 2009 by United Press International.