PHILADELPHIA -- With every start, Roy Oswalt is getting closer to getting back to the postseason.
Oswalt (13-13) allowed one run and six hits, striking out seven. The right-hander, who lost his first start after he was acquired from Houston on July 29, is 7-0 with a 1.55 ERA in nine games since his rocky Philadelphia debut.
"These guys have been there two years in a row. I'm trying to get back there," Oswalt said. "I almost feel like a rookie again. Sometimes you get in a rut with the same team. We weren't winning."
The Phillies won their fifth straight game and reduced their magic number for clinching their fourth consecutive division title to 12. The two-time NL champions lead Atlanta by three games.
"We're doing the right things right at exactly the right time," manager Charlie Manuel said.
Marquis (2-9) retired the first batter and exited after the next eight reached safely. He was charged with six runs and six hits, increasing his ERA from 6.60 to 7.71. That's not what the Nationals expected when they gave the 2009 All-Star a $15 million, two-year contract over the winter.
Oswalt has exceeded expectations. The three-time All-Star was 6-12 with Houston, mainly because the Astros averaged 2.3 runs in his starts. Oswalt has a 1.94 ERA in 10 starts with the Phillies, and they're 9-1 in those games.
"My expectations of pitchers is high," Manuel said. "I'm tough on pitchers because I want them to pitch good. He's been everything I expect him to be."
Oswalt was moved up a day and pitched on regular four days' rest. The Phillies adjusted their rotation so Cole Hamels, Roy Halladay and Oswalt will face the Braves in a three-game series that starts Monday. The trio also would be in line to pitch in a season-ending three-game set at Atlanta, if needed.
"The pitchers are starting to feed off each other," Oswalt said.
Singles by Placido Polanco and Ryan Howard sandwiched around a walk to Chase Utley loaded the bases with one out in the first. Werth drove in a run with an infield single. Ibanez hit a two-run single to make it 3-1, and advanced to second on the throw to third.
After Ruiz was hit by a pitch, the Phillies got a big break. Wilson Valdez hit a sharp grounder toward second baseman Danny Espinosa that should've been an inning-ending double play. Instead, the ball hit umpire Tim Tschida and ricocheted into the outfield for an RBI single.
"It could've been a couple runs, instead they got six," Nationals manager Jim Riggleman said.
Oswalt then lined an RBI single to left, chasing Marquis. Shane Victorino's fielder's choice grounder made it 6-1.
"They're obviously a good lineup," Marquis said. "I've got to do a better job getting ahead of hitters."
Ryan Zimmerman's RBI single in the first accounted for Washington's only run.
The six runs were the most in the first inning by Philadelphia this year. ... The Phillies are an NL-best 258-167 after the All-Star break in six seasons under Manuel. ... A crowd of 45,338 was the 115th straight sellout at Citizens Bank Park. ... Victorino extended his hitting streak to 10 games. ... Oswalt is 63-16 in August, September and October. ... Zimmerman finished 4 for 4.