HOUSTON -- Washington Nationals manager Frank Robinson called it the lowest point of his team's rapidly unraveling season.
"We haven't been to this low of a point before," Robinson said. "We just keep going and going and it doesn't seem like we're capable of stopping it and turning it around."
Washington fell three games behind the Astros in the NL wild-card race.
The Nationals had hoped their series with the Astros -- the start of a 13-game road swing -- would help turn around a disatrous second half.
But the Nationals instead lost their fifth game in their last six outings, again struggled to find offense and suffered through another aimless outing from Ryan Drese (3-6), who lost fifth consecutive start.
Pettitte (10-8) struck out five in eight innings and allowed four hits, his last a two-run homer to Preston Wilson in the seventh.
Since losing to Baltimore on June 14, Pettitte is 7-1 and has allowed 10 earned runs. The left-hander has lasted at least seven innings in his last six starts.
"Whenever I get in trouble I feel like I'm staying within myself a lot better and making ptiches instead of trying to power guys," Pettitte said. "But mechanically I feel good, and I think that's why my location has been so good."
Brad Ausmus had a three-run double in the sixth that extended Houston lead to 6-0. It was another timely hit for the catcher, who has dramatically improved his swing after batting just .214 through June.
Ausmus is been batting .302 since July, which he credits to hitting coach Gary Gaetti moving his hands lower and farther back on the bat.
"I've kind of stuck with that and it's felt a lot better," Ausmus said. "It's certainly the most important difference."
Drese, making his second return to Texas since the Rangers waived the opening day starter in June, allowed six runs on nine hits.
Drese left in the sixth after Pettitte singled, one batter after Ausmus doubled to right field with the bases loaded.
"He's just not consistent," Robinson said of his embattled pitcher. "You don't know what you're going to get from him from hitter to hitter or from pitch to pitch."
Lance Berkman also doubled and drove in two runs for the Astros, who have won three of four and improved to a major league-best 38-16 at home.
Jose Guillen, making his first start in five games after sitting out with a sore left shoulder, hit his 20th homer in the ninth for Washington.
Washington is 3-10 on the road since the All-Star break and still has 10 games remaining on its 13-game trip. The Nationals are 7-19 overall since July 14.
"We can't find ourselves," Guillen said. "It's pretty ugly."
- Astros rookie Willy Taveras had his major league-leading 58th infield hit. He also extended his hitting streak to eight games.
- The Nationals are 1-6 against left-handers since July.
- Pettitte's single was his fourth hit of the season, tying a single-season career high.
- Rockets guard Tracy McGrady attended the game.