Top seed Roger Federer [Images] was beaten 6-4, 7-6 by American James Blake in the quarter-finals of the Olympic men's singles on Thursday.

The Swiss produced an error-strewn display under the Centre Court lights after a long rain delay and was outplayed as Blake claimed his first victory against Federer in nine meetings.

Defeat continued Federer's miserable season and leaves the 27-year-old running out of chances to win an Olympic medal, although he is still in the doubles here.

Last month he lost his Wimbledon [Images] title to Rafael Nadal [Images] and he will cede his world number one ranking to the Spaniard on Monday.

Blake said he always believed he could break his duck against the 12-times grand slam champion.

"I always believed in myself," Blake, who is now just one victory away from a guaranteed medal, told reporters. "I'd lost to him eight, nine, 10, 50 times, I don't know how many, but I had the feeling it could be my day today."

Eighth seed Blake played two superb baseline rallies to close out the first set as Federer served at 5-4 down.

Federer then fell 3-0 down in the second set before battling back to 3-3.

Luck looked to be on Federer's side when a nervy volley dribbled over off the net tape when Blake was two points away from victory in the 12th game of the second set.

However, the inspired American would not be denied, surging 6-2 ahead in the tiebreak and sealing victory on his first match point.

"He didn't play a good first game and that put me up straight away. I played loose and free and I always fancy my chances on the 5-4 game," Blake said.

"When he got that net cord I started wondering if it would turn around and he would come back playing like the best player in the world. But I know I can beat the best in the world on my day and today I proved it."

Blake will play either Chile's Fernando Gonzalez or Frenchman Paul-Henri Mathieu for a place in the final.

Venus beaten

Later in the evening, double Olympic champion Venus Williams [Images] was beaten 7-5, 7-5 by China's Li Na in the quarter-final of the women's singles.

Venus, the seventh seed, crashed out to an inspired home favourite who immediately offered the victory as a present to the Beijing [Images] Olympics.

"Venus was the best player in the world, she has just won Wimbledon," Li told reporters. "She has big serves that I can't receive but I told myself to calm down and play my best.

"Wow! You can't see this kind of crowd anywhere else. It helped me and pushed me a lot. This is a present I give to the Beijing Olympic Games but I have no time to celebrate. I have more matches to go."

Venus raced into a 3-0 lead in the opening set but Li, the first Chinese player to reach the top 20 in the world rankings, proved she had the stomach for a fight.

After regaining parity, she seized the moment to break the Williams serve at 5-5 and served out the set in style.

Williams, who won gold in doubles and singles in Sydney eight years ago, founds herself 3-0 down in the second set as the decibel level reached new heights.

The American fought back to lead 4-3 but Li continued to play the better tennis and claimed victory when her opponent dumped a forehand into the net on her second match point.

Serena exits

Earlier in the day Serena Williams [Images] lost 3-6, 6-4, 6-3 to Russia's [Images] Elena Dementieva at the Olympic Tennis Centre.

Williams, playing in her first Olympic singles tournament here despite winning doubles gold in Sydney, appeared on course for victory when she powered through the opening set but her challenge evaporated.

"I didn't serve well, it was what is was," fourth seed Williams told reporters. "It hasn't sunk in yet."

Dementieva, a singles silver medal winner at the Sydney Olympics [Images], gradually came to terms with her opponent as she found her range with her precision baseline game.

The Russian, who has been known to suffer from nerves at crucial moments throughout her career, outbattled Williams to break serve in the second game of the final set and romped away to a 5-0 lead.

Williams recovered one break of serve and then saved two match points at 2-5. Dementieva kept her nerve, however, and closed out the match at the third attempt in the following game when Williams volleyed wide.

"It was a very exciting moment," Dementieva told reporters. "This is the biggest tournament for me this year."

Dementieva now faces an all-Russian semi-final with Vera Zvonareva [Images] who beat Austria's Sybille Bammer 6-3, 3-6, 6-3.