Isner has suprised everyone with his efforts in Washington

The 6ft 9in American fired 30 aces in a 6-4 6-7 7-6 win and next faces Gael Monfils, who beat Marat Safin 6-3 7-5.

Isner, 22, claimed his first ever ATP win when he beat Tim Henman in round one and all four victories this week have come in final set tie-breaks.

"My serves are going well and it's a great weapon," said Isner.

"To beat a player of Tommy's calibre is unbelievable. I was fortunate to get through this."

Isner led Georgia to the US college team crown and lost the individual final in May before winning challenger events in June and July.

Haas, playing in his first event since pulling out of Wimbledon with a torn stomach muscle, joked: "The tour should come up with a system where if you're over 6ft 6in, you can't play.

"He has a big game. That serve is a huge weapon."

Another player with a huge serve, top seed Andy Roddick, beat South Korean Lee Hyung-Taik for the second week in a row to move into the semi-finals.

The American, champion in Washington in 2001 and 2005, fell 5-4 behind in a first set tie-break before taking it 8-6 and broke Lee twice in the second to wrap up a 7-6 6-2 victory.

"That's by far the best I've hit the ball at this tournament," Roddick said. "I felt like I was controlling my service games and even controlling the tempo when I was down. It definitely gave me confidence."