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Roy Oswalt is set to make his Colorado debut on Thursday, as the Rockies take on the Washington Nationals in the opener of a four-game series at Nationals Park.

Oswalt, a three-time All-Star, fizzled out with the Rangers last year and was ultimately bumped to the bullpen after nine shaky starts. He looked a bit more like vintage Oswalt in five starts with Double-A Tulsa, as the 35-year-old veteran posted a 2.16 ERA before getting the call to the big club.

"He's always a competitive guy, a tough at-bat," manager Walt Weiss said. "He's a nice piece to have. He's ready to go. His velocity is real good and throwing all his pitches. He's done just about everything in this game, pitched in big games and been a staff ace."

The last time Oswalt pitched in Washington, he threw eight shutout innings and fanned nine batters while with the Phillies on Aug. 20, 2011.

Taking the hill for Washington is Jordan Zimmerman, who is seeking his 10th win of the season. The right-hander is 6-0 with a 1.06 ERA in seven home starts. However, he was tagged for six runs in Cleveland on Saturday but the Nationals bailed him out by rallying for a 7-6 win.

Washington avoided a series sweep in Philadelphia on Wednesday night thanks to an 11th-inning grand slam from Ian Desmond, the first of his career. Former Phillies right fielder Jayson Werth drove in the other two runs in the Nats' 6-2 victory, including the tying single in the ninth inning off Phillies closer Jonathan Papelbon.

"Ian is quite a character. He got a lot of big hits for us in the past," manager Davey Johnson said. "The only thing that I worry about sometimes is that he tries too hard. It seemed like he was a little too geared up, and then when he got to two strikes, he shortened up and he crushed that ball. (Phillies reliever Michael) Stutes threw a good breaking ball and (Desmond) crushed it, didn't try to swing hard."

Despite the dramatic victory, the Nationals entered Thursday a game below .500 and 7 games back of Atlanta in the NL East. They are also 6 1/2 games out in the wild card race.

The Rockies have struggled of late, dropping six of their last eight to fall 2 1/2 games off the pace in the NL West. They suffered a three-game sweep in Toronto this week as they were held to a mere five runs total for the series.

On Wednesday, starter Juan Nicasio served up a three-run home run to Adam Lind in the first inning, and the Rockies went on to lose, 5-2. Nicasio did not get much help from his offense, although Carlos Gonzalez hit his National League- leading 21st homer and Michael Cuddyer extended his hitting streak to 17 games. The Rockies lead the NL in hits.

These teams met up last week in Colorado, where the Nationals won two of three.