There were five errors in the contest accounting for three unearned runs.
Cincinnati went 14 innings before losing to the Chicago Cubs on Thursday and it appeared the Reds were headed toward another marathon on Friday.
Bruce, however, ended it by sending a 1-2 pitch to left that barely cleared the wall. It was his 11th home run of the season.
Milwaukee tied it on an eighth-inning home run by Martin Maldanado, but the Brewers suffered only their second loss in six games.
The win went to Alfredo Simon (5-2), who worked the top of the 10th. Burke Badenhop (0-3) gave up the home run to Bruce.