(CNN) -- Top seed Roger Federer saw his Olympic challenge come to a crashing end on Thursday with a straight-sets defeat at the hands of American James Blake while both the Williams sisters also suffered shock defeats in the women's singles
James Blake celebrates his shock quarterfinal victory over top seed Roger Federer in Beijing.
The outgoing world number one was unable to find his rhythm in their quarterfinal clash, slipping to a 6-4 7-6 loss to Blake, who will now face Chile's Fernando Gonzalez in the last four.
Gonzalez beat Frenchman Paul-Henri Mathieu in straight sets while Rafael Nadal and Novak Djokovic will face each other in the other semifinal.
In the women's singles, fourth seed Serena Williams also exited at the quarterfinal stage, losing 3-6 6-4 6-3 to Russian Elena Dementieva.
She was then joined on the sidelines by Wimbledon champion Venus, who went down 7-5 7-5 to Li Na of China in their last eight match.
The home crowd erupted with the biggest roar of the tournament when Li hit a winner on match point. She gained a single break in each set to pull off a massive upset as Venus misfired on her forehand.
Earlier, Serena, beaten by her sister in the Wimbledon final, was no match for fifth seed Dementieva, despite a brief rally in the deciding set.
She bravely staved off two match points as she trailed 5-2 in the deciding set, but Dementieva held at love in the next game, sealing the victory when Williams pushed a volley wide.
Minor consolation for the Williams duo is that they remain in contention for a doubles medal and play Ayumi Morita and Ai Sugiyama of Japan in a second round match on Friday.
The start of the tennis schedule had been delayed by over three hours due to a series of heavy downpours in Beijing.
Federer, who is enduring the worst season of his career, was broken when serving for the first set and crushed 7-2 in the second-set tie-break, ending his bid for a first Olympic medal.
It was Federer's third defeat in six matches since losing his five-year Wimbledon crown. The 27-year-old Swiss will be replaced by Nadal on Monday after four-and-a-half years as world number one.
Blake, wearing a red headband to go with his white shirt and blue shorts, started with intent and Federer needed a service winner to stave off a break point in game eight.
The American earned another chance at 5-4 when Federer went long after a pulsating rally. The Swiss 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 capitalized 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, pummeling the Swiss with his big forehand, raced to a 4-1 lead.
He put away his first match point when Federer's service return went long, dropping to his knees and screaming in delight. Blake had lost all eight of their previous matches, winning only one set in five years.
The American missed the 2004 Games during a traumatic spell when he broke his neck, lost his father to cancer and developed shingles. Federer finished fourth in 2000 but was shocked in round two four years ago early in his reign as world number one.
After the delays, it was past midnight local time when Nadal thrashed Austria's Jurgen Melzer 6-0 6-4 and Djokovic saw off Gael Monfils of France 4-6 6-1 6-4.
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