World number one Roger Federer [Images] survived a ferocious claycourt battle with Spanish qualifier Nicolas Almagro at the Rome Masters on Friday, winning 6-3, 6-7, 7-5 to set up a semi-final with fourth seed David Nalbandian.

Holder Rafael Nadal [Images] extended his winning streak on clay to 51 matches by cruising past Chile's Fernando Gonzalez 6-4, 6-3 in the other half of the draw.

Nadal next meets Frenchman Gael Monfils, who upset fifth seed Andy Roddick [Images] 6-2, 6-3 in a damp squib of a match during which the American received treatment for a sore shoulder that hampered his game.

It was a nervy afternoon for Federer, who is at his most vulnerable on clay and has yet to win this title in five previous visits.

Federer looked comfortable when he broke in the second set after winning the first. But a double fault on his first match point opened the way for 20-year-old Almagro to come storming back to level.

The Swiss player looked tense in the deciding set before Almagro wavered when serving to take the match into a deciding tiebreak.

"I'm prepared for a three-hour match...I have to be," Federer told reporters of his semi-final with Nalbandian, one of the few players with a winning record over the world number one.

"I play many matches that don't go on that long but I have to be prepared for the unexpected.

GREAT MATCH

"I had a chance to win in the second set, I double-faulted, then he almost came back and beat me, so I got lucky today. It was a great match."

Federer can expect another battle with Nalbandian, who has won six of their 10 previous meetings.

The Argentine has dropped only one set in his last nine matches on clay, a run that includes winning last week's title in Estoril, and was rarely stretched during a 6-3, 6-3 triumph over Croatian Mario Ancic.

Most of the buzz in the Italian capital this week has centred on the prospect of another showdown between Federer and world number two Nadal.

Nalbandian, however, said the two men are beatable.

"They aren't out of reach, even if they are playing really well at the moment," said the world number three.

"It's right to respect them but I believe our respect is becoming another weapon for them."

The way Nadal is playing suggests he needs little help from anyone.

Once the 19-year-old Spaniard found his rhythm against Gonzalez there was only going to be one winner.

"I was expecting a difficult match so I was very concentrated," said Nadal.

If he wins his next two matches, Nadal will equal the record of 53 consecutive victories on clay set by Argentine Guillermo Vilas in 1977.

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