Roger Federer's reign as the world No1 will end after four and a half years if Rafael Nadal, the man who beat him in the French Open and Wimbledon finals, wins the Cincinnati Masters at the weekend.

Federer's position became vulnerable last night when he was beaten 7-6, 4-6, 7-6 by the towering Croatian Ivo Karlovic in the third round of the tournament.

Spanish star Nadal, meanwhile, eased into the last eight with 6-4, 7-6 victory over German Tommy Haas.

Federer's exit, following his first-round defeat last week in Toronto, also means that British No1 Andy Murray will not have to face him in the semi-finals if he continues his own run at the tournament.

Murray was in impressive form as he defeated Russia's Dmitry Tursunov 6-3,6-3 to set up today's quarter-final clash with Carlos Moya. But he found conditions tough and said: "It's so hot, so humid. I was very happy to come through in two sets. I changed the pace of the ball well and used the short slice and he didn't like that very much.

"My calf got a little bit tight at the end of the match. It's unbelievably hot here, it's normal for your muscles to get a little bit tired, but I feel good and I'm looking forward to my match with Carlos Moya. I have played Moya once on hard courts and won pretty comfortably."

Karlovic hit 22 aces against Federer, who has been the world's top-ranked player for a record 235 consecutive weeks. Federer said: "It's not easy coming from slower conditions to fast ones but I have played Ivo on all surfaces and he actually gets a greater play on the slower ones.

"But what are you going to do if he serves like that? Maybe if I got the first set I would be sitting here and it would be different, but that's not the case."

It could also be all change at the top of the women's rankings after world No1 Ana Ivanovic suffered a shock 6-2, 1-6, 6-2 defeat at the hands of 17-year-old Austrian Tamira Paszek in the Montreal Cup. Fellow Serbian Jelena Jankovic, who reached the last eight with 6-3, 6-2 demolition of Canada's Stephanie Dubois, will take over No1 spot if she makes it into Sunday's final.