CINCINNATI -- Shin-Soo Choo got the Reds offense going early and they opened up a big lead.
They needed every bit of it against the Diamondbacks.
The Reds opened an 8-0 lead through four innings, but had to hold on tight at the end.
"It was the epitome of a full moon," Reds manager Dusty Baker said. "We were riding easy 8-0, then all the sudden it was 8-4. A lot of stuff happened. Those guys [Diamondbacks] didn't give up.
The Reds opened up a six-game lead over Arizona in the race for the National League's second wild-card spot.
Cincinnati remained third in the NL Central, but moved to 2½ games behind first-place Pittsburgh, which lost at San Diego.
Mike Leake allowed four runs in the fifth inning, but was dominant in his other five innings for first win in his past five starts.
Leake (11-5) allowed six hits and four runs with no walks and three strikeouts while the Reds were building an 8-0 lead.
"I was mixing and matching and keeping them off balance," Leake said. "Two pitches got me. Sometimes that happens. Fortunately, we had an eight run lead. It ended up being a stressful game."
The Diamondbacks added three runs against the Cincinnati bullpen before Aroldis Chapman finished for his 32nd save and first career two-inning save.
Baker was forced to take Jonathan Broxton out of the game in the eighth inning with soreness in his elbow. Broxton spent five weeks on the disabled list, returning on Aug. 6.
"It was disheartening to go get Broxton," said Baker, who went to his closer Chapman an inning early. "I went out with [pitching coach] Bryan Price. BP said, 'He's done.' "
The umpire asked Baker, whom he wanted and he indicated that he wanted Chapman. That answer prevented Baker from making a double-switch with the pitcher's spot due up third in the bottom of the eighth.
"That's the first time I've ever experienced something like that," Baker said. "I was so worried about Broxton."
Frazier fought off a tough pitch and had a hit that landed out of the reach of shortstop Didi Gregorius.
"It was an ugly swing with good results," Baker said.
Frazier was as surprised as anyone the ball fell for a hit.
"It's crazy," Frazier said. "I was talking to [Wil] Nieves at the plate about what a game it was. We were laughing at each other. It's humbling. It's weird. It's confusing at times.
"No matter what you get a 'W' at the end, you're happy. I don't know what I was doing man. I was confused. He (Brad Ziegler) has a nasty slider. It started two feet behind me and I thought that's going to come back in. I kind of jumped out of the way. My body did its own thing and away it went."
Choo hit Brandon McCarthy's second pitch of the game 337 feet down the left field line, where the ball appeared to hit the lens of a TV camera just inside the foul pole.
The home run was Choo's 16th of the season, his sixth leadoff home run of the season and 11th of his career.
The Reds had four consecutive two-out hits to open up a 4-0 lead in the second. Leake lofted a ground-rule double down the right field line that bounced into the netting above the visitor's bullpen to drive in one run, and Choo drove in two more with a single to center.
Chris Heisey added an RBI double and Devin Mesoraco made it 7-0 with a two-run single up the middle through a drawn-in Arizona infield to knock McCarthy (2-8) out of the game after a season-low 2 1/3 innings.
He allowed eight hits and a season-high seven runs with one walk and two strikeouts while slipping to 0-5 in his past seven starts.
Choo narrowly missed hitting his second home run of the game leading off the fourth inning, settling for a double off the top of the left-field wall and scoring on Joey Votto's single.
The Diamondbacks broke through against Leake with four runs in the fifth. Pinch-hitter Jason Kubel delivered a two-run single and Adam Eaton followed on Leake's next pitch with his second homer of the season, a 367-foot drive into the visitors' bullpen.
Prado added a run-scoring single off J.J. Hoover in the seventh and Gerardo Parro led off the eighth with his ninth home run, a fly ball off Jonathan Broxton that hit the top of the left-field wall and bounced back onto the field before the umpires ruled it a homer after a 3:02 replay delay.
"We played our tails off," Arizona manager Kirk Gibson said of his offense. "They did a heck of a job."
Chapman pitched more than one inning for the first time this year. He also struck out in his first major league at-bat.
"He has to work on his bunting," Baker said. "We let him swing away. He's a good athlete. He can swing the bat and is probably one of our fastest players. We hoped that he would hit one in the gap so people could see it."
- Chapman struck out in the eighth in his first career plate appearance.
- Broxton, who missed almost two months this season with an elbow injury, walked off the field with a Reds trainer after facing two batters in the eighth.
- Cincinnati LHP Tony Cingrani is scheduled to make his regular start on Sunday despite leaving Tuesday's game in the fourth inning with a lower back strain.
- Reds RHP Johnny Cueto, who's on his third stint on the disabled list with a strained muscle behind his right shoulder, played catch Wednesday, the first time he has thrown since going on the DL on June 29.
- RHP Trevor Cahill (4-10), who snapped a six-game losing streak with a victory in Pittsburgh in his previous start, is Arizona's scheduled starter in Thursday's series finale against RHP Mat Latos (12-4).