The Giants jumped off to a 4-0 lead in the first, thanks in part to Posey's two-run blast, and his grand slam in the fourth extended San Francisco's lead to 11-1.
Andres Torres homered and drove in three runs, Freddy Sanchez chipped in with three RBI and Aubrey Huff added a solo shot for the Giants, who won their third straight over the Brewers.
Tim Lincecum (9-4) tossed seven strong innings, striking out 10 while allowing one run on four hits.
Brewers starter Chris Narveson (7-6) was lit up for 10 runs on nine hits in 3 1/3 innings.
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