"He's been very consistent for us," said Arizona manager Bob Melvin, who also has used Petit as a reliever. "He was on the corners all night. The ball was down. He mixed his pitches."
The Diamondbacks had 12 hits and stretched their division lead over Los Angeles to three games after the Dodgers lost 9-2 at Philadelphia.
"We knew coming into the game. We saw that they lost," Diamondbacks first baseman Chad Tracy said. "It makes you want this thing even more."
The Marlins are 4½ games behind the Phillies for second place in the NL East.
Petit was effective because he kept the ball down, using a deceptive slider that broke down and away. He got ahead early on most of Florida's hitters and looked more comfortable as he steamed through the innings.
"He jumps at you with long strides," Melvin said. "He hides the arm a little bit. His changeup looks just like his fastball. It can be up in the zone, which can look like a fastball and get early swings on it."
Young put the Diamondbacks ahead 2-1 in the fifth, curling a one-out shot into the left-field stands near Arizona's bullpen for his 18th home run.
It came off Florida starter Chris Volstad (4-3), who allowed four runs and seven hits in 5 1/3 innings.
"The guy had good stuff," Tracy said. "I think after the first time around we just kind of dialed in on him."
Arizona stretched the lead to 4-1 in the sixth when Mark Reynolds scored on a groundout by pinch-hitter Tony Clark, the first batter to face reliever Mark Hendrickson. Stephen Drew added a two-out RBI single.
At that point, Melvin didn't want to pull Petit. But the manager felt he needed to build on the 2-1 lead, so in went Clark.
"I was feeling good enough to throw another inning and TC comes in with the bases loaded, gets an RBI," Petit said. "I can't control that. I went six innings. Anything to help the team win."
"The radar gun only says 86 or 88 but he's a lot more sneaky than that," Baker said. "He kind of turns his body a little bit. He's got a little bit of a pause, then all of a sudden out comes the ball."
The Diamondbacks added three runs in the eighth against reliever Renyel Pinto.
"I'm not trying to hit a home run there," said Montero, who has hit safely in seven of the last eight games and his hitting .520 over that stretch. "I'm just looking for a good pitch and trying to use the whole field."
Florida second baseman Dan Uggla was charged with two errors on one play for bobbling a grounder by Drew and making a poor throw that allowed Salazar to reach third. Salazar scored on Young's sacrifice fly for a 7-1 lead.
"They got a couple of runs that inning and we could have been out of there and still seen a 2-1 ballgame," Marlins manager Fredi Gonzalez said. "But then it just kind of got out of hand."
The Diamondbacks went to work right away against Volstad, taking a 1-0 lead on Adam Dunn's RBI single in the first. It followed consecutive singles to start the game by Drew and Young.
Ramirez tied it for Florida when he hit his 28th homer into the left-field stands. He was back in the lineup after a one-game break to rest a bruised right thumb.
- Two-time Olympic beach volleyball gold medalist Misty May-Treanor, wife of Marlins C Matt Treanor, got a standing ovation when she was introduced on the video board. She dangled her gold medal from the Beijing Games.
- Reynolds, playing 3B, made a highlight-reel stab to stop a liner by Jorge Cantu in the fourth, then made the throw to first from his knees.
- Marlins RF and longtime Diamondbacks standout Luis Gonzalez received a long cheer when he batted in the first. He was hitless in three at-bats.
- Florida 1B Mike Jacobs snapped an 0-for-28 skid with a seventh-inning single.
- The Marlins are 6-2 against Arizona this season.