Bruce smacked a pitch from Marco Estrada (2-6) into the right field stands to break a 4-4 tie, finishing the game 3-for-4. The Reds sent 10 men to the plate in the big frame to win going away.
Ramon Hernandez added a two-run shot and an RBI single, and Joey Votto stroked a run-scoring single among his three hits as Cincinnati rebounded after two straight losses to the Brewers.
Bill Bray (2-1) earned the win with a scoreless bottom of the ninth inning.
Yuniesky Betancourt had an RBI single for Milwaukee.
Brewers starter Shawn Marcum lasted six innings, yielding three runs on six hits.