MASON, Ohio — Covering the court on a balky knee that only appeared to bother him once, Andy Murray pulled off a breakthrough win.

Murray repeatedly ran down shots and hit on-the-move winners today during a 6-4, 6-4 victory over Ivo Karlovic that advanced him to the championship of a Masters Series tournament for the first time.

The 21-year-old Murray will play either Novak Djokovic or Rafael Nadal, who has turned the Cincinnati Masters into a seismic event. By reaching the semifinals, Nadal has guaranteed he will overtake Roger Federer as the world's No. 1 player in the next few weeks.

If Nadal wins the $2.6 million ATP Western & Southern Financial Group Masters, he will move up to the top spot in Monday's rankings. Given the way points accrue — the last 52 weeks are counted — he will surpass Federer no later than the Aug. 18 rankings.

Federer has been No. 1 for a record 235 consecutive weeks. He and Nadal have been ranked 1-2 since July 25, 2005. Now, the 22-year-old Nadal will become the third Spaniard to hold the No. 1 spot, joining Carlos Moya (1999) and Juan Carlos Ferrero (2003).

Karlovic made it possible.

The 6-foot-10 Croat was in a deep slump heading into the tournament, having lost his last three matches. The fast courts in Cincinnati have suited his hard-to-track serve.

Karlovic put together the most surprising run of the tournament — and pulled off its biggest upset. He knocked off Federer in three sets Thursday, opening the way for Nadal to move ahead of the Swiss star if he advanced deeper in the tournament.

Playing in his first Masters series semifinal, Karlovic met his match. Murray's game was solid all-around, preventing Karlovic from getting comfortable. Karlovic had 32 unforced errors, Murray only nine.

During his first four matches, Karlovic held serve in 49 of 50 games, the only failure coming against Federer. Murray broke him twice in the opening set to establish the tone.

Karlovic had only four aces during the match. He fought off seven break points in the opening game of the second set before Murray finally broke through for a 1-0 lead that gave him the upper hand.

It's been an encouraging summer for Murray, the top-ranked British player at No. 9 in the world. At Wimbledon, he reached a Grand Slam quarterfinal for the first time. He also reached the semifinals last week in Toronto before losing to Nadal.

His biggest concern has been his suspect right knee. An irregular kneecap causes swelling and pain from time to time, and all the matches on hard courts last week left him with some inflammation. He had a medical scan on the knee last Monday to make sure there was nothing seriously wrong.

Murray twisted the knee on a shot in the quarterfinals Friday, leaving him with a limp for several minutes. He moved fluidly on the court Saturday, which allowed him to keep Karlovic off-balance. He twisted the knee slightly during the second set and had to take a few seconds to stretch it out.