Beijing:Top seed Roger Federer and Venus and Serena Williams all went crashing out of the Olympic Games quarter-finals today as the tournament was rocked by major upsets.
James Blake secured his first win over Federer 6-4, 7-6 (7/2) to increase the Swiss maestro's woes, while Serena Williams' campaign ended at the hands of Russia's Elena Dementieva 3-6, 6-4, 6-3.
Venus followed with a stunning reverse against China's Li Na, ranked 42, who raised the Centre Court roof with a 7-5, 7-5 win.
Federer, who hands over his number one crown to Rafael Nadal on Monday, was broken when serving for the first set and was crushed in the second-set tie-break.
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 number one 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."
For Federer, it was his third defeat in six matches since losing his five-year Wimbledon crown to Nadal five weeks ago.
The 12-time Grand Slam winner has spent a record four-and-a-half years as world number one.
Venus Williams' defeat was even more unthinkable given her Wimbledon win last month, but the 2000 doubles and singles champion wilted in a pressure-cooker atmosphere created by thousands of noisy fans.
Li becomes the first Chinese to reach an Olympic semi-final where she will play either top-ranked Jelena Jankovic or Russian sixth seed Dinara Safina.
Number four seed Serena Williams, also one of the clear favourites after a series of pull-outs, was seeking her first singles medal after winning doubles gold in 2000 but missing Athens 2004 injured.
After a blistering start she appeared to run out of steam, double-faulting to hand over the second set and quickly going 5-1 down in the decider.
"I didn't serve really well. It was what it was. It hasn't sunk in yet," said the glum American.
Federer's dismal start to the American hardcourt season, with defeats to Gilles Simon and Ivo Karlovic, has given his rivals hope and Blake, wearing a red headband with his white shirt and blue shorts, started with grim intent.
Federer needed a service winner to stave off a break point in game eight but Blake earned another chance at 5-4 when the Swiss went long after a pulsating rally.
The top seed then produced an incredible leaping backhand from behind the baseline but dunked his next shot into the net to go a set down.
Federer lacked his usual aggression, looking content to rally and misfiring with the serve and forehand. Blake capitalised by forcing three break points in game two of the second set, going ahead when Federer put a backhand long.
The scare seemed to spark Federer into action and he broke back at 1-3 when Blake swiped a backhand into the tramlines.
As tension mounted, Federer held off Blake in two service games to force the tie-break. But the aggressive Blake, pummelling his big forehand, raced to a 4-1 lead and put away his first match point when Federer's return went long.
Blake will face either Fernando Gonzalez or Paul-Henri Mathieu in the semi-finals. Nadal is playing Austria's Jurgen Melzer and third-ranked Novak Djokovic has Gael Monfils in the other men's quarter-finals.
Dementieva, who also won Olympic singles silver in 2000, will play her ninth seeded compatriot Vera Zvonareva for a place in the final after she beat unseeded Austrian Sybille Bammer 3-6, 6-3, 6-3.
"I'm very excited for the moment. I'm very excited about my semi-finals here and I don't want to think about a medal," Dementieva said.
Play started some three hours late after heavy rain hit the competition for a second day.