SAN DIEGO - With one well-placed curveball, Roy Oswalt changed the complexion of the game and helped the Houston Astros take the wild-card lead.

Oswalt struck out Xavier Nady with runners on first and second and no outs in the sixth inning, dodged a bases loaded jam and led the Astros to a 6-2 victory over the NL West-leading San Diego Padres on Monday night.

Oswalt, who won his 15th game, and three relievers combined on a five-hitter, and Lance Berkman hit a two-run homer for the Astros, who moved a half-game in front of Philadelphia and Florida in the wild-card race.

The Padres fell to 61-63 but continued to lead the division by four games over Arizona, which lost 4-1 to the New York Mets. This is the latest a team has led a division with a losing record.

Oswalt (15-10) held the Padres to two runs and five hits in six innings to win for the first time in five starts.

“My last three starts, I’ve been grinding through it, and I was able to keep it low-scoring,” Oswalt said.

Holding a 3-2 lead, Oswalt got into trouble in the sixth by allowing a leadoff single to Brian Giles and walking Mark Loretta. Nady had an 11-pitch at-bat, working the count full before fouling off four straight pitches, including one that catcher Brad Ausmus dropped.

Nady then struck out looking at a 62 mph curveball that replays showed was outside.

“What could have been a disaster, he stopped it,” Astros manager Phil Garner said. “It didn’t look so good, and he battled to get out of the inning.”

Said Oswalt: “Three-two, I threw him a few fastballs that he fouled off, then I threw a curveball over that I started high enough that he gave up on it and I was able to drop it in there for a strike.”

Padres manager Bruce Bochy disagreed with the call.

“He’s got no chance to hit that pitch,” Bochy said. “That ball didn’t come near to catching any part of the plate. That’s a huge call. He had bases loaded with nobody out versus first and second and one out. Definitely changes the game.”

Damian Jackson then popped weakly to second. Oswalt hit Olivo to load the bases to bring up pinch-hitter Robert Fick, who grounded weakly to first on the first pitch.

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Oswalt struck out five and walked three. He improved to 6-1 in his career against San Diego. He also beat the Padres on a five-hitter on July 4, 4-1 at Houston.

Relievers Chad Qualls, Dan Wheeler and Russ Springer held the Padres hitless over the final three innings.

Berkman homered to right-center field on a 3-2 pitch from reliever Rudy Seanez with one out in the seventh, his 13th, for a 5-2 lead. Willy Taveras was aboard on a leadoff single. Berkman also hit an RBI double in the ninth, giving him three hits and three RBIs.