Federer and Williams sisters crash out

BEIJING (Reuters) - Three of the biggest names in tennis, Roger Federer and sisters Venus and Serena Williams, crashed out at the quarter-final stage of the Olympics in a night of shocks on Thursday.

Federer's nightmare year plumbed new depths when he lost to American James Blake, a player he had beaten in all eight of their previous meetings, after torrential rain had delayed the start of the quarter-finals until late evening.

The Swiss top seed's game went totally haywire against Blake and he lost 6-4 7-6.

With news of Federer's shock defeat reverberating around the Olympic tennis complex, Serena Williams became the second of the night's big casualties when she lost to Russian fifth seed Elena Dementieva 3-6 6-4 6-3.

Home favorite Li Na then stole the show, out-hitting double Olympic champion Venus Williams in a sensational 7-5 7-5 victory that sparked noisy celebrations on the floodlit Centre Court.

"Wow! You can't see this kind of crowd anywhere else. It helped me and pushed me a lot," Li, the world number 42 but a one-time member of the top 20 club, told reporters.

"This is a present I give to the Beijing Olympic Games but I have no time to celebrate. I have more matches to go," said the 26-year-old.

Li still does not know who her semi-final opponent will be as the quarter-final between Jelena Jankovic and Dinara Safina was postponed when rain began falling again at 1.30 a.m. on Friday just as organizers hoped to catch up on the match program.

SPARK MISSING

Federer had been banking on an Olympic gold medal to ease the pain of defeat by Spain's Rafael Nadal in the Wimbledon final last month but when he needed the old spark to dig himself out of trouble it was nowhere to be seen.

Blake sensed his chance from game one when the errors had already started flowing from Federer's racket.

"I always believed in myself," Blake, 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."

Not even a netcord when he served to stay in the match at 5-6 could save 12-times Grand Slam champion Federer, who will lose his world number one ranking to Nadal on Monday.

Blake now plays Athens singles bronze medalist Fernando Gonazalez of Chile, who beat France's Paul-Henri Mathieu 6-4 6-4.

Second seed Nadal slipped through almost unnoticed amid the carnage, thrashing Austria's Jurgen Melzer 6-0 6-4 to set up a tasty semi-final with Serbian third seed Novak Djokovic who beat Frenchman Gael Monfils 4-6 6-1 6-4.

Predictions of a Williams monopoly of the women's gold and silver medals in Beijing were turned on their head as both produced erratic performances against inspired opponents.

Serena, playing in her first Olympic singles tournament despite winning doubles gold in Sydney eight years ago, powered through the opening set but her challenge evaporated.

Dementieva surged into a 5-0 lead in the final set and although her nerves began to fray as Williams mounted a late salvage operation, she hung on.

"It was a very exciting moment," Dementieva, looking to go one better than her silver in Sydney, told reporters. "This is the biggest tournament for me this year."

She will face Vera Zvonareva, who beat Austria's Sybille Bammer 6-3 3-6 6-3, in an all-Russian semi-final.

(additional reporting by Liu Zhen)

(Editing by Ralph Gowling)