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 (7/0), 6-3 while third seed Haas ousted Bulgarian 11th seed Grigor Dimitrov 7-6 (7/5), 7-6 (7/3).[break]
"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 better."
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 $1.76 million hardcourt event, a tuneup for the US Open.
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 tennis.
"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 (7/0), 3-6, 6-3.
"I couldn´t find my rhythm at all," Kerber said. "It was not actually my game."
Isner fired 18 aces to defeat Cyprus´ Marcos Baghdatis 6-7 (5/7), 6-4, 6-4 while Tursunov outlasted Australia´s Marinko Matosevic 6-3, 4-6, 7-6 (7/4).
"I was pretty confident even though I lost the first set," Isner said. "I know I´m not going to win every tie-breaker. I was in a good rhythm and serving well.
"I wanted to make him beat me. I didn´t want to lose my mind out there and I held it together pretty well over the last two sets."
It 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.
"I´m very tired but I´ll keep fighting," Rybarikova said. "It´s not going to be easy for me. For sure she will be more fit. She only played one game. She´s the favorite."
The other women´s semi-final will match French fourth seed Alize Cornet -- who ousted Romanian fifth seed Sorana Cirstea 7-6 (7/5), 6-2 -- against Germany´s Andrea Petkovic, who eliminated Argentina´s Paula Ormaechea 4-6, 6-1, 6-3.
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