Roger Federer's wretched season hit a new low here when his dream of winning gold in the singles was shattered as he was beaten 6-4, 7-6 by James Blake.

Having lost his Wimbledon crown and his world No1 ranking in a year beset by illness and loss of form, the crestfallen Swiss saw victory at the Games as more than just a major consolation but rather the focus of his campaign.

Instead, though, the patchy form which he has taken with him into the hardcourt season betrayed him once again as Blake, a man who'd never beaten him in eight attempts, pulled off an extraordinary upset in their quarter-final.

"This is unbelievable," said the American. "It's not very often that you get to say beat the best player in the world any time but to do it at the Olympics with USA on your chest makes me incredibly proud.

"Maybe if you play him often enough he's bound to have an off-day. One day I'll be able to tell my grandkids that I beat the best of all time."

Federer's slump was highlighted by the fact he made 56 unforced errors.

Blake said: "Roger didn't play his best but I felt I served well especially in that second set tie-breaker when I needed it most. Normally he plays the big points best but today I did that."

Blake will now Frenchman Paul-Henri Mathieu or Chile's Fernando Gonzalez in the semi-finals while Federer, whose love of the Olympics was made obvious here when he carried the Swiss flag at the opening ceremony, is still clinging on to his dream of gold as he is still in the quarter-final of the doubles with Stanislas Wawrinka.

There was also a shock in the women's event as Serena Williams lost to 2000 silver medalist Elena Dementieva of Russia 3-6, 6-4, 6-3.

Seeded No4, the American overcame two match points during an 18-point game to hold for 5-3. But Dementieva held at love in the next game, sealing the victory when Williams pushed a volley wide.