"Petit looked pretty good," Arizona manager Bob Melvin said. "Two days off after throwing three innings and giving us six innings like that. That's pretty impressive. He keeps the ball down, throws strikes, and gets ahead of guys. It's amazing what you can do when you can locate your fastball and keep it down and then mix in your offspeed on top of that. Every time we've seen him, he's done that."
The Arizona Diamondbacks are now 47-38 and 2-3 on their current 10-game road trip, with two more games left in St. Louis before heading to Cincinnati. The Cardinals fell to 37-43.
After throwing three innings for Triple-A Tucson on Friday at Sacramento, Petit (1-1) allowed just one run on five hits in 5 1/3 innings to pick up his second career Major League victory.
"I didn't think I'd be able to get what I did out of him, to tell you the truth," Melvin said. "With two days off like that, we were going to monitor him and see how he's doing after the fourth. With his pitch count where it was, we could stretch him out a little bit. He gave us exactly what we needed."
Petit threw first-pitch strikes to 16 of the 22 batters he faced. He struck out three and walked two.
"I was feeling good," Petit said through an interpreter, his catcher, Miguel Montero. "I started to throw strikes. I went with the best pitch, the fastball, tonight. I didn't try to do too much, [just] whatever I know how to do. I expanded the zone, kept the ball down. That was it."
Petit was acquired in March when the D-backs traded right-handed reliever Jorge Julio and cash considerations to the Florida Marlins. Petit, who was signed by the Mets in 2001, was traded from New York to Florida in 2005 as part of the deal that sent Carlos Delgado to the Mets.
"Since I've known him, he's always thrown a lot of strikes," Montero said. "The best thing he's got is his command. ... If you've got a pitcher who can command his fastball, you're going to see a lot of results."
Melvin's lineup shake-up worked early on Tuesday.
The D-backs got two unearned runs in the first inning off Cardinals starter Todd Wellemeyer. Center fielder Chris Young, who reached base on an error to lead off the game, scored on third baseman Mark Reynolds' sacrifice fly to right field. Second baseman Orlando Hudson scored on shortstop Stephen Drew's two-out single to right-center.
Young hit a solo home run, his 12th of the season, in the second inning.
In the third inning, first baseman Chad Tracy hit a two-run homer and Reynolds followed with a solo homer for a 6-0 lead. The home runs, the sixth of the season for each player, marked the first time this year the D-backs have hit back-to-back home runs.
"When we get on guys early, it seems like it takes some pressure off our pitchers and puts a little pressure on their offense to try to make something happen," Tracy said. "[Petit] was throwing strike one and when you get strike one, you can expand a little bit. He did a great job. Hats off to him."
Wellemeyer (2-1) took the loss. He allowed six runs (four earned) on seven hits in three innings.
"I felt good, I threw some good pitches, but any mistake, those guys are all over it," Wellemeyer said. "They are an aggressive team, I'm an aggressive pitcher. Sometimes it works in your favor, sometimes it doesn't."
Petit ran into trouble in the sixth. With shortstop Aaron Miles on base following a fielder's choice, Petit surrendered a single to left to first baseman Albert Pujols -- Pujols' third hit off Petit on the night -- and then walked left fielder Chris Duncan to load the bases.
Melvin brought in reliever Tony Pena, who gave up a sacrifice fly to right field by third baseman Scott Rolen to make it 6-1 and then retired right fielder Juan Encarnacion to end the threat.
"That's what he does for us," Melvin said of Pena, who then pitched a perfect seventh inning. "Typically he comes in and gets the toughest outs. He's been involved in almost every one of our wins, whether it's coming in in the seventh or the eighth or coming in in that exact situation that he came in today. ... He's an invaluable piece for our team and one of the better guys at his role in all of baseball."
The D-backs added a run in the eighth when pinch hitter Conor Jackson drove home Drew with a single to center field as a fireworks display was going off above the Mississippi River.
Right-hander Dustin Nippert threw two scoreless innings in the eighth and ninth to secure the victory and snap the D-back's three-game losing streak.