Milwaukee wasted a chance to leave Cincinnati with a four-game split.
Chris Heisey scored from first base on Todd Frazier's 10th-inning double into the left-field corner, and the Reds rallied to beat the Brewers 4-3 Sunday.
Milwaukee went 3-4 on a trip that began in St. Louis but still has the major leagues' best record at 21-11.
"This would've been an important game to win," Brewers manager Ron Roenicke said. "If you lose, you lose the road trip. If you win, you win the road trip. You wouldn't think one game could do that, but it does."
Chris Heisey drew a two-out, four-pitch walk from Tyler Thornburgh (3-1), and Frazier followed with a line drive that one-hopped the left-field wall. Heisey beat shortstop Jean Segura's off-target relay to the plate.
"That was a big series," said Brewers starter Kyle Lohse, who was in line to improve to 5-1. "They played us tough. They pitched really well."
Milwaukee has gone 14 straight games without scoring more than five runs.
"We've got to do a better job of battling," Roenicke said. "If it makes sense to shuffle, we'll shuffle, but I don't see where there are pieces to make those changes, so we have to do a better job. We get opportunities, and we're not getting the job done right now."
Brandon Phillips greeted reliever Brandon Kintzler with a tying 425-foot drive to center with one out in the eighth, Phillips' second homer this season and first since April 5.
Sam LeCure (1-1) pitched the 10th for the win.
Lohse allowed a season-high eight hits and two runs in 6 1-3 innings, including Brayan Pena's home run.
"It was a weird outing for me," Lohse said. "At first, they were super aggressive. I tried to use that, and they started taking. They made me work for every out I got."
Cincinnati starter Alfredo Simon, who hadn't allowed more than two runs in any of his first five starts, gave up three runs and five hits in seven innings.
Scooter Gennett hit Simon's third pitch of the game 357 feet into the right-field seats for his second homer of this season. The Brewers made it 2-0 in the third on Jonathan Lucroy's two-out, RBI single.
Pena hit the first pitch of the third inning 456 feet over the right-field wall for his third homer of the season, all in the last five days. He left for a pinch runner with a tight hamstring in the seventh.
Khris Davis restored the two-run lead with a solo homer in the fourth. Cincinnati again made it a one-run game in the bottom half on Frazier's sacrifice fly after Phillips led off with a double - his fourth hit in five at-bats since getting Friday off.
NOTES: Davis has 15 home runs in his two-year big league career, including five against the Reds. ... Price expects LHP Aroldis Chapman to be activated for the Reds' homestand that is scheduled to start Friday. Chapman is due to take Sunday and Monday off before starting and pitching an inning for Triple-A Louisville on Tuesday. He is to throw an inning of relief on Wednesday. Chapman is coming back from surgery to repair injuries to his left eye sustained when he was struck by a line drive during a spring training game on March 19.