INDIAN WELLS, California (AFP) - World number one Roger Federer advanced to the semi-finals of the Pacific Life Open without hitting a shot Friday as Tommy Haas withdrew with a sinus infection.

Federer was due to take on the unseeded German in the quarter-finals on Friday afternoon.

It was another setback for Haas, a former world number two who is now ranked 36th in the world.

The 29-year-old German has been trying to battle back after undergoing a third shoulder operation in November.

This week he had surprised sixth-seeded American Andy Roddick in the second round and beaten 11th-seeded Briton Andy Murray in the fourth.

"I deeply regret that I have to withdraw from the Pacific Life Open. I was playing my best tennis of the season, and was looking forward to playing Roger this afternoon," Haas said in a statement released by the organisers.

"I am sure the tennis will be incredible this weekend, and look forward to returning again next year."

Federer, who is seeking his first title of 2008 after a bout of mononucleosis contributed to an uncharacteristically slow start to the year, will meet the winner of the quarter-final contest between seventh-seeded Argentinian David Nalbandian and unseeded American Mardy Fish, ranked 98th in the world.

Haas actually withdrew from his last scheduled meeting with Federer, in the fourth round at Wimbledon last year, with a torn abdominal muscle.

"I am very sorry for Tommy as I was looking forward to another great match with him," said Federer, who has won eight of 10 meetings with the German. "We have played quite a few times, and I know and respect Tommy a lot. I hope he gets better quickly."