Juan Martin Del Potro sets up Washington ATP final with John Isner

Top seed Juan Martin Del Potro and US eighth seed John Isner advanced to the final of the ATP and WTA Washington Open with rain-interrupted triumphs on Saturday.

Seventh-ranked Del Potro, the 2009 US Open champion from Argentina, defeated German third seed Tommy Haas 7-6 (7/4), 6-3 while Isner blasted 29 aces in a 6-7 (7/9), 6-3, 6-4 victory over Russia's Dmitry Tursunov.

Del Potro, the 2008 and 2009 Washington winner, was down 1-4 when rain halted the first set for more than three hours, but won the next four games once play resumed.

"The rain delay helped me play much better in the second part of the match," Del Potro said. "I came back with too much power on my forehand. It was too much for Tommy."

Del Potro saved a set point with a second-serve ace in the 12th game and took the tie-breaker when Haas hit a backhand long.

"He went for it. There's nothing you can do," Haas said of the second-serve ace.

"The rain delay was annoying. Bothered me a lot. Momentum was on my side. (The first set) would have been mine."

Haas netted a backhand to surrender a break in the eighth game of the second set and Del Potro held for the victory, stretching his win streak to 13 matches at the $1.76 million hardcourt event, having not played in the US capital since his title run four years ago.

"It's amazing after four years to return to the final," Del Potro said.

Haas fell to 0-5 against Del Potro and has never taken a set from the South American star.

Isner, who won last week in Atlanta, took the only break of his second set on the first point after a rain delay of more than an hour in the fourth game, hitting a forehand winner.

"It was just about getting warmed up and winning that point," Isner said. "I did come out fresher from that rain delay. Winning that first point gave me all sorts of momentum."

In the final set, Isner broke at love in the fifth game, fired four aces in a row to sweep the final game and 10 aces overall.

"In the third set, with his serving, I would like to see somebody do better than I did," Tursunov said. "You really can't see the serve too well. It's just a reaction. It's really a lot of guessing and hoping you put your racquet where the ball is going to be."

Isner, ranked 20th, tops the season ATP rankings in aces with 587 in 37 matches and has a tournament-best 78 aces this week.

"I've served incredibly well the last two weeks," Isner said. "That has carried me. I'm in a pretty good serving rhythm right now. I'm riding high with a lot of confidence."

But he is 0-3 all-time against Del Potro, having never taken a set.

"Against Isner, anything can happen," Del Potro said. "I have to be careful with my serves and find a way to take one of his. Sometimes you get frustrated when you can't return his serve.

"He has improved his game. He is never missing shots from the baseline. He moves like a small guy and that makes him dangerous."

Del Potro, playing his first event since losing an epic Wimbledon semi-final to top-ranked Novak Djokovic, seeks his 15th career ATP crown and second of the year after Rotterdam in February. His most recent final was at Indian Wells, where he lost to Rafael Nadal.

Isner seeks his eighth career ATP title and third crown of the year, having also won in April at Houston.

Defending women's champion Magdalena Rybarikova, the seventh seed from Slovakia, beat Russian third seed Ekaterina Makarova 6-2, 6-0 and will face Germany's Andrea Petkovic, a 7-5, 6-3 winner over French fourth seed Alize Cornet.