The patient Spaniard, who has chased the Swiss for three seasons in a good-natured rivalry, guaranteed he will claim the world number one ranking this month with a 7-6 (7/3), 6-1 victory over Ecuador's Nicolas Lapentti on Friday.

By reaching the semi-finals, Nadal is assured of overtaking Federer on August 18 at the latest. If he can carry on to claim the title on Sunday he can claim number one on Monday, with the Beijing Olympic tennis competition due to start six days later.

Federer has held the world number one ranking for more than four years - his reign so far lasting 235 straight weeks. But his hopes of holding off Nadal faced with his third-round loss here to Croatian Ivo Karlovic.

"I'm so happy to be getting number one," said Nadal, the winner of 30 career titles including five grand slams. "I fought hard to get here. I was also happy to be number two, I won a lot of matches. It's been tough, I have a lot of matches on my back. The conditions are tough here. It's been tough to play well after winning Toronto last week."

Nadal will become the 24th player in the history of the ATP Rankings to hold the number one position and the third Spaniard to accomplish the feat, joining Carlos Moya (1999) and Juan Carlos Ferrero (2003).

He faces a semi-final challenge against third-seeded Novak Djokovic after the Serb defeated former tennis school friend Ernests Gulbis 6-3, 6-4.

"I'll have to play better than tonight, I need to play my best-ever match at Cincinnati," Nadal said. Lapentti, ranked 89th after standing inside the top 10 in 1999, last played a Masters Series quarter-final in Miami six years ago.

Djokovic shelved any sentiment as he dispatched Gulbis to reach his fifth Masters Series semi-final. The young pair - at 21 Djokovic is two years older than Gulbis - spent time at the Niki Pilic academy in Munich and fondly remember their tearaway teenaged years.

"It was a matter of time when he was going to make this breakthrough and just gain the experience with the time," said Australian Open winner Djokovic. "Obviously he's showing his quality now. He's one of the young rising stars that we expect to see more in the future."

Djokovic rode one break per set to improve his record over Gulbis to 2-0. At the top of the draw, Andy Murray beat Carlos Moya 2-6, 6-3, 6-1 to complete a spirited fightback. The eighth-seeded Scot will test his confidence against the mighty serve of Karlovic, who notched 24 aces in his 7-6 (8/6), 7-6 (7/3) win over German Philipp Kohlschreiber. Murray goes in with full confidence after reaching his fifth career semi-final at the elite Masters level.

"I think I can win a Masters Series, and I'd love to do it this week," he said. "I played well last week and had a tight match against Nadal. This week I'm playing well again. You know, hopefully I can do it."