The seventh-seeded Croat saved a match point with an ace when trailing 6-5 in the third-set tiebreaker, then clinched the match with another ace at 8-7.

Karlovic, who won his first two ATP titles this year and earned a career-high ranking of No. 25 this week, will play Tommy Haas in Saturday's semifinals at the Royal Tennis Hall.

The fourth-seeded Haas defeated Juan Monaco 6-3, 6-3 to reach the semifinals for the second time since 2004.

"I expected to have some good rallies. He's a great athlete on the court and puts a lot of balls in play," Haas said. "I like playing indoors. I'm happy with my results so far here, but the next match will be difficult. Karlovic is a tough player."

Haas, who won the first five games, has struggled somewhat recently, getting crushed by Igor Andreev in the Davis Cup semifinals against Russia and exiting in Bangkok in the second round last week.

Karlovic, at 6-foot-10 the tallest player on the ATP tour, had 30 aces for a tour-leading 1,093 this year. Goran Ivanisevic set the record in 1996 with 1,477 aces.

Karlovic lost only four points with his serve in the first set, when his first-serve percentage was 88 percent.

Clement saved two break points in the second set at 1-1, and then broke Karlovic for the only time in the match to lead 4-3 when the Croat's forehand sailed long.

There were not service breaks in the third set.