DUBAI, United Arab Emirates — Andy Roddick used his strong serve to upset Rafael Nadal 7-6 (5), 6-2 today and reach the semifinals of the Dubai Tennis Championships.

The sixth-ranked Roddick closed the match with one of his 17 aces, and had numerous service winners against the second-ranked Spaniard. One ace was 149.75 mph, breaking the Dubai serve record he set a day earlier.

"I knew I was going to hit it well, from the first point. I kind of let it fly, and that's what it took tonight," Roddick said. "Anything less than that and I wasn't going to win."

Roddick had several love games in the match, using his powerful forehand to keep Nadal at bay.

"The way he was serving tonight, it's tough," Nadal said. "You feel a lot of pressure when you're serving."

Nadal said he made too many errors on his forehands on crucial points.

"Against a big server you have to put these balls in if you want to win," Nadal said. "It's disappointing for me today."

Also today, Nikolay Davydenko edged Andy Murray 7-5, 6-4 to reach the semifinals.

Murray, who had upset top-ranked Roger Federer in the first round, was forced out of his defensive game by the fifth-ranked Russian's hard serves and quick moves on the baseline.

"I played well," said Davydenko, who trailed 3-0 in the second set. "If you are crazy, you can lose your match. ... If you are concentrated you can win every game."

Murray opened the tournament by extending Federer's losing streak to two matches.

"I'm not terribly disappointed with the week as a whole," Murray said.

Feliciano Lopez also advanced to the semifinals, defeating fourth-ranked David Ferrer 6-4, 6-3.

Third-ranked Novak Djokovic, the Australian Open champion, faces Igor Andreev later today.