Fielder had a pair of two-run homers and a solo shot to lead the Brewers to their 10th win in 14 games.
Reliever Jared Hughes (0-1) was tagged with the loss after giving up a two-run blast to Fielder that broke a 4-4 deadlock in the seventh inning.
Alex Presley and Neil Walker both had two hits for the Pirates. Presley also drove in a run and Walker scored the first run of the game on a single by Matt Pagnozzi in the second inning.
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