Fiers (4-4) broke a personal three-start winless streak, handling the Nationals on four hits while walking three.
The rookie right-hander allowed only one runner to advance as far as second base as the Brewers evened their four-game set with Washington at one victory apiece.
Ross Detwiler (5-4) was touched for four runs on seven hits in a 4 2/3-inning effort for National League East-leading Washington.
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