The Williams sisters both crashed out of the Olympic singles tournament Thursday, Venus losing to unseeded local hope Li Na in a shock defeat and Serena succumbing to Elena Dementieva.

Venus Williams went down 7-5, 7-5 to Li, ranked 42nd in the world, in the pressure cooker environment of a raucous Center Court that would have had tennis "purists" shuddering.

Li, the first Chinese woman to reach the world's top 20 and the first to win a WTA event, will play either world No 1 and top seed Jelena Jankovic of Serbia or Russian No 6 seed Dinara Safina in the semifinals.

Li Na of China celebrates after defeating Venus Williams of the US during their 2008 Beijing Olympic Games women's singles quarterfinal match at the Olympic Green Tennis Center in Beijing yesterday. Li Na won 7-5, 7-5. [Agencies]

Serena Williams' bid for a first Olympic singles gold also went up in smoke after she lost to Russian No 5 seed Elena Dementieva in the quarterfinals.

No 4 seed Williams, who won doubles gold with her sister Venus at the Sydney Games but missed Athens four years ago through injury, went down 6-3, 4-6, 3-6 to this year's Wimbledon semifinalist.

"I didn't serve really well," said Williams. "It was what it was. It hasn't sunk in yet."

Dementieva, who also won Olympic singles silver in 2000, will play her ninth-seeded compatriot Vera Zvonareva for a place in the final after her 6-3, 3-6, 6-3 defeat of unseeded Austrian Sybille Bammer.

"I'm very excited about being in the semifinals of the Olympic Games," said Dementieva. "I felt like I was playing well today and I hope to continue playing like that.

"She started really very well. I was just trying to stay positive, trying to fight and I had nothing to lose. I was trying to play more aggressive and that was the key to the match."

Men's top seed Roger Federer also crashed out of the Games quarterfinals.

James Blake of the US secured his first win over Federer 6-4, 7-6 (2) to increase the Swiss maestro's woes.

Federer, who hands over his No 1 crown to Rafael Nadal on Monday, was broken when serving for the first set and was crushed in the second-set tiebreak.

Blake, 28, collapsed to his knees and roared with delight as he celebrated the win, which he described as a career highlight. He had taken only one set off Federer in nine attempts stretching back to 2003.

"I would say beating the No 1 player in the world has got to be up there," Blake said.

"The feeling, the emotion involved is huge. It's something I have to not think about right now because there's still work to be done in this tournament."

AFP