Zack Cozart hit two homers for the Reds, whose offense has reawakened for 22 runs in two games in Milwaukee after manager Bryan Price went on a profanity-laced tirade in his pregame media session Monday.
Mike Fiers (0-3) was shelled for eight runs in four innings for Milwaukee, which lost its seventh straight. The Brewers have the worst record in the majors at 2-12.
For a change, the offense wasn't the main problem for the Brewers. They broke out for their highest-scoring game of the season despite a lineup littered with backups because of injuries.
Elian Herrera hit a grand slam in the sixth to get the Brewers to 13-10.
The Reds had more punch. Cozart led off the top of the seventh with a solo shot.
Balls were flying inside Miller Park, where the roof was closed on a chilly spring evening. The teams combined for seven homers, including five for Cincinnati.
Eight-place hitter Cozart now has three homers in two games and four on the season -- matching his output for all of 2014.
But the middle of the order set the tone at the plate.
Frazier's homer in the fourth to left off Fiers made it 8-4, answering a four-run rally the previous inning by Milwaukee.
Jason Rogers had two throwing errors at third for the Brewers. The first error off Brandon Phillips' bouncer in the third inning proved especially costly after the next batter, Bruce, hit his fourth career grand slam to open the scoring.
The game took on the feel of a Saturday softball game by the middle innings.
Cozart finished 3 for 5 with three RBIs and three runs. Herrera had five RBIs and scored three times.
Jason Marquis (1-1) got the win despite giving up nine hits and five runs in five innings. He had eight strikeouts.
Reds: Catcher Devin Mesoraco made his first appearance since coming off the bench on April 12 against St. Louis. Mesoraco has been dealing with a sore hip. He walked as a pinch hitter in the seventh and scored a run.
Brewers: Manager Ron Roenicke said C Jonathan Lucroy could miss four to six weeks with a broken left toe. He got hurt on Monday. ... Scooter Gennett went on the DL on Tuesday after the second baseman cut his hand in the shower on Sunday in Pittsburgh. Herrera was called up from Triple-A to replace him on the roster.
Reds: Johnny Cueto (0-2) hopes the offensive revival continues for his start on Wednesday. The 22 runs combined over the two straight victories exceeded the team's 19-run output during a 1-7 stretch preceding the trip to Milwaukee.
Brewers: Jimmy Nelson (1-1) is as close as the struggling Brewers have to a stopper these days. Milwaukee starting pitching had an ERA of 5.96 entering Tuesday, the worst in the majors.