Since trailing the Cubs by a game on Aug. 4, Milwaukee has gone 8-11 and slid to third in the NL Central. Guerra (6-9) has been stuck in his own slump, going 0-4 in his last eight starts since July 8.
The Reds ended a five-game losing streak by taking advantage of Guerra, who gave up six runs and faced only 13 batters in his shortest outing of the season. Peraza had two of his four hits off Guerra, who threw 45 pitches.
"That first inning was hard, it was long," Guerra said through a translator. "I was in the (strike) zone and they attacked me, they were going after it. There's no excuse."
Peraza homered and Guerra let in a run with a wild pitch during a nine-batter first inning. Guerra left with the bases loaded and one out in the second. Guerra has pitched four innings or less in four of his last eight games.
Asked whether Guerra will remain in the rotation, manager Craig Counsell was noncommittal.
"We're going to have to talk about that," Counsell said. "He's been struggling a little bit since the All-Star break."
Anthony DeSclafani (7-4) gave up four runs in 6 2/3 innings, including Yelich's solo homer. Yelich added a three-run shot in the seventh off Jared Hughes. Yelich has 10 homers in August and a career-high 25 overall.
The Brewers have homered in 20 consecutive games at Great American Ball Park, the longest streak by any team at the park, which opened in 2003.
DeSclafani missed the first two months recovering from a strained oblique. In five August starts, he's gone 3-1 with a 2.62 ERA.
"It's been the same plan since I've been back: throw strikes and get contact," DeSclafani said. "You're going to have good stretches and bad stretches. I'm feeling pretty good about my last four outings."
Raisel Iglesias got the final out in the eighth. He gave up a pair of hits in the ninth and Yelich's run-scoring groundout -- his career-high fifth RBI -- before closing out his 24th save in 28 chances.
The Reds held a moment of silence pregame for Sen. John McCain, who died of brain cancer.
The Reds are 54-60 since April 19, when they replaced Bryan Price with interim manager Jim Riggleman.
CATCHING A BREAK
Eugenio Suarez headed toward the dugout after reaching third base in the second inning, thinking the Brewers had pulled off a double play to end the inning. Instead, there were only two outs. Suarez was allowed to return to third base because Counsell had called timeout, unaware of Suarez leaving the base.
Brewers: RH Zach Davies is expected to rejoin the team over the weekend. He's been recovering from a shoulder injury since June. Counsell hasn't decided whether to use him in the rotation or the bullpen.
Reds: 1B Joey Votto took ground balls and ran on the field pregame. He's expected to have another workout on Wednesday, and the Reds will decide whether to activate him. Votto has been sidelined since Aug. 17 by a sore right knee, which was hit by a pitch on Aug. 4.
Brewers: Freddy Peralta (6-4) makes his second straight start against the Reds. He got a 4-0 win last Wednesday, allowing three hits and fanning seven in seven innings.
Reds: Matt Harvey (6-7) makes his 19th start for the Reds. He's 1-0 with a 1.45 ERA in his last three.
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