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Argentina's top-seeded Juan Martin Del Potro booked a semi-final matchup against Germany's Tommy Haas with a straight-set victory on Friday over Kevin Anderson at the ATP and WTA Washington Open.

Seventh-ranked Del Potro, the 2009 US Open winner and the Washington champion in 2008 and 2009, beat the South African seventh seed 7-6 6-3 while third seed Haas ousted Bulgarian 11th seed Grigor Dimitrov 7-6 7-6.

"It's a good opportunity to reach another final but I have to beat a very good player," Del Potro said. "He's playing good this season and he has got a lot of experience. I'll try to play like today or even betterphone covers."

US eighth seed John Isner, who won his seventh career title last week in Atlanta, will face Russia's Dmitry Tursunov in the other men's semi-final at the $US1.76 million ($A1.98 million) hardcourt event, a tuneup for the US OpenWigs for Cancer Patients Bright Hopes.

Del Potro, in his first event since losing an epic Wimbledon semi-final to top-ranked Novak Djokovic, has won all four prior ATP matches against Haas without losing a set, including a 2008 Washington semi-final and most recently in the round of 16 this year at Indian Wells.

"Del Potro is one opponent I despise playing in many ways," Haas said. "He has given me a lot of trouble. And he has been playing terrific tenniscloud computing hong kong.

"It's going to be a challenge. I hope I can mix it up, play aggressive and not give him too many points."

Del Potro, who never faced a break point in the match, rolled through the first-set tie-breaker, winning it at love when Anderson netted a backhand.

That was part of a run of 15 points in a row by the South American, including the first six of the second set on his way to the first break of the match for a 2-0 lead.

Del Potro held serve to the finish, which came after one hour and 45 minutes on a service winner, and improved to 5-0 all-time against Anderson.

"I just tried to return his serves and find a way to break," Del Potro said. "In the second set I broke very early and played with more confidence the rest of the match."

Women's top seed Angelique Kerber of Germany was ousted by defending champion Magdalena Rybarikova 7-6 3-6 6-3.

Isner fired 18 aces to defeat Cyprus' Marcos Baghdatis 6-7 6-4 6-4 while Tursunov outlasted Australia's Marinko Matosevic 6-3 4-6 7-6.

Their upcoming match will be the first ATP meeting for Tursunov and Isner.

"I'm not looking forward to facing him," Tursunov said. "He has been serving well lately. He has a lot of confidence right now."

Tursunov, ranked 61st, seeks his first semi-final since February at Marseille. That was his first appearance in the last four since winning in 2011 at s-Hertogenbosch.

Slovakian seventh seed Rybarikova advanced to a Saturday semi-final against Russian third seed Ekaterina Makarova, who barely had won her opening game when Romania's Monica Niculescu retired with a left wrist injury.

The other women's semi-final will match French fourth seed Alize Cornet - who ousted Romanian fifth seed Sorana Cirstea 7-6 6-2 - against Germany's Andrea Petkovic, who eliminated Argentina's Paula Ormaechea 4-6 6-1 6-3.