WASHINGTON: US top seed Andy Roddick outlasted John Isner 6-7 (3/7), 6-2, 7-5 to book a finals date with defending champion Juan Martin Del Potro at the ATP Washington Classic.

Fifth-ranked Roddick, playing in his first event since a five-set Wimbledon final loss to Roger Federer, fired only nine aces to 20 for Isner but hit 69 percent of his first serves and made just two unforced errors in the third set.

"Every service game I held was a challenge and he eventually just broke me down," Isner said.

It took five break-point chances in the third set before Roddick finally broke Isner in the penultimate game, then held serve to win after two hours and 31 minutes.

"I wasn't converting but it's not because I wassn't putting the ball on the court. He was coming up with the goods," Roddick said. "All you can do is put yourself in position and hope you get that point."

Second seed Del Potro of Argentina defeated Chilean fourth seed Fernando Gonzalez 7-6 (7/2), 6-3 in a showdown of South America's top players in the other semi-final at the 1.4 million-dollar hardcourt tournament.

Sixth-ranked Del Potro, who won his only prior match with fifth-ranked Roddick in last year's Los Angeles final, will try to become the first back-to-back Washington champion since Andre Agassi in 1999.

"I will have to play much better than today," Del Potro said. "He has improved a lot since we played."

Roddick, 26, seeks his 28th career crown and fourth in Washington after taking the title in 2001, 2005 and 2007, when he beat Isner, 24, in the final. It would be his second ATP title of the year after a February win at Memphis.

Del Potro, 20, seeks his sixth career title and second of the season after winning at Auckland in January.

Isner saved two break points in the opening game of the third set and one each in the ninth and 11th games before Roddick closed him out. Roddick took a tumble diving for a return in the ninth game but was unhurt.

"I'm fine," Roddick said. "I'm worried about the court."

Roddick saved two break points in the opening game of the match and Isner swatted forehand winners to deny Roddick on a break point in both the eighth and 12th games.

Isner won five of the last six tie-breaker points - three of them on drop volleys, including the clincher - to take the first set.

Roddick opened the second set by breaking Isner with a backhand winner and broke again in the seventh game on his way to forcing a third set.

Del Potro fired 10 aces in hot and humid conditions on his way to beating Gonzalez for the first time in four meetings between the 2009 French Open semi-finalists.

"It was tough for both of us. The court was so hot," Del Potro said. "After the first set Fernando was so tired. Me too. It was very difficult for both of us. We were very tired and the sun was very strong."

In the second game of their second set, Gonzalez fell trying to turn quickly and stayed on the ground until Del Potro climbed over the net to help him up. Three points later, Gonzalez miss-hit a forehand to surrender a break.

"I was thinking lay down with him. If I did we would never stand up," Del Potro said. "He told me, 'Come with me and give up.' But I didn't. We kept playing like professionals."