A three-run sixth inning cost the Brewers in their first game back from the All-Star break.
Milwaukee led 3-1 thanks to RBI singles by Jonathan Lucroy and Jonathan Villar, as well as a solo home run from Scooter Gennett.
But the Reds got to starter Matt Garza in the home half of the sixth to take the lead and would go on to win 5-4. Garza allowed four runs on six hits in five plus innings to fall to 1-3.
Villar and Gennett each finished with two hits. Chris carter had a home run taken away from him in the fourth thanks to a leaping catch by Jay Bruce.
The series continues Saturday with a 6:10 p.m. first pitch in Cincinnati. Jimmy Nelson opposes John Lamb.