Second-seeded Roger Federer has pulled out of his quarterfinal against James Blake at the Paris Masters because of a back injury.

"My back has been stiff for the last couple of days and I woke up this morning and it did not respond to the treatment I had last night," Federer said on his Web site yesterday. "As a precaution and because I would be unable to play at 100 percent tonight, I have unfortunately had to take the difficult decision not to play."

Federer apologized to the disappointed fans who came to watch him at the Bercy indoor arena.

Federer said he hopes the injury will only take a few days to recover and that he will be able to play in the season-ending Masters Cup in Shanghai which begins on 9 November.

The 11th-seeded Blake now advances to the last four, where he will face either No. 7 Andy Roddick or No. 13 Jo-Wilfried Tsonga.