LONDON — Rafael Nadal overcame Ivo Karlovic's powerful serve to reach the Queen's Club semifinals, where he will face defending champion Andy Roddick.

The top-seeded Nadal defeated Karlovic 6-7 (5), 7-6 (5), 7-6 (4) today. Roddick, seeded third, advanced without picking up a racket when Andy Murray withdrew with a thumb injury.

Also reaching Saturday's semifinals was No. 2 Novak Djokovic, who won 6-2, 6-2 over four-time tournament champion Lleyton Hewitt.

Neither Nadal not Karlovic managed a service break in their match. Nadal didn't even bother reaching for many of the 35 aces hit by the 6-foot-10 Croat. Nadal had just two break points in the match.

"It was a very tough match," Nadal said. "It's a very important win because I beat a specialist on this surface, a big server. I think I returned well, but I didn't have lot of chances.

"On grass, especially these matches, everything is decided in two balls," the French Open champion said. "So it is very important to be prepared, and have the best concentration."

Roddick, who returned to action this week after missing the French Open with a shoulder injury, had reached the quarterfinals when fellow American Mardy Fish quit with an ankle injury after the first set of their third-round match.

The sixth-seeded Murray said he sprained the thumb when it got caught in the net during Thursday's match against Latvia's Ernests Gulbis.

"I woke up this morning, couldn't pick up my pillow, I was struggling to pick up my phone," Murray said. "Came down here and tried to practice, hit some balls. I could hit the ball decent, but I couldn't hit a backhand volley, and slicing was very sore."

Murray said he expects to recover before Wimbledon, which starts June 23.

"If I played today I still think I would have been OK to play Wimbledon. (But) there's no point going on the court feeling 50 percent," he said. "I don't think it's going to take too long. I just have to take it day to day and see how the swelling goes down."

Djokovic broke Hewitt's serve in the opening game of the match and got another service break to lead 4-1. In the second set, Djokovic again took an early lead when he broke for 2-1, and then held off three break points at 3-2 before breaking again for 5-2.

"I played probably the best tennis of my life on grass," Djokovic said. "I was returning a lot of serves, being very aggressive and patient at the same time, serving a very high percentage, having a lot of aces. I put him under pressure. He made some unforced errors, crucial unforced errors, in the important moments."

Djokovic next will play fourth-seeded David Nalbandian, who defeated Richard Gasquet 6-4, 3-6, 7-6 (3).