Roger Federer's bid for his first Olympic singles medal ended Thursday when he lost to James Blake of the United States, in a match where the Swiss star swatted stray balls in frustration and hung his head and stomped behind the baseline.

That was the start of an upset parade in the quarterfinals. Serena Williams lost to Elena Dementieva of Russia. And as the clock approached midnight, Venus Williams was beaten by Li Na of China.

The startling sequence came in a tournament that had gone mostly according to form through three rounds. But upsets have long been the norm in Olympic tennis -- since 1988, no top-five player has won the gold medal in men's singles.

Federer won't do it this year. With the sort of lackluster performance once unthinkable for the stylish Swiss, he was eliminated 6-4, 7-6 (2).

The No. 5-seeded Dementieva, who won a silver medal in Sydney in 2000, raced to a 5-0 lead in the final set and held on to beat Serena Williams 3-6, 6-4, 6-3. The unseeded Li then delighted a partisan center-court crowd by eliminating Venus Williams 7-5, 7-5.

A few players went against the upset trend. No. 2 Rafael Nadal beat Jurgen Melzer 6-0, 6-4 in a match that ended at 1:08 a.m. Nadal's semifinal opponent will be No. 3 Novak Djokovic, who rallied to defeat Gael Monfils 4-6, 6-1, 6-4.

No. 3 Novak Djokovic was in jeopardy but rallied and reached the semifinals by beating Gael Monfils 4-6, 6-1, 6-4 in a match that ended at 12:50 a.m.

Top-seeded Mike and Bob Bryan of the United States also went against the upset trend, winning their quarterfinal match in doubles against Lleyton Hewitt and Chris Guccione of Australia, 6-4, 6-3.

Blake's semifinal opponent will be No. 12 Fernando Gonzalez of Chile, who defeated Paul-Henri Mathieu of France 6-4, 6-4.

The No. 8-seeded Blake, a first-time Olympian at 28, was the lone U.S. male to survive the first round of singles. He had won only a single set in eight previous matches against Federer.

"If you play him enough times, he's bound to have an off day," Blake said.

But the top-seeded Federer has been battling a yearlong slump that has left him stalled at 12 major titles, two shy of Pete Sampras' record. His Wimbledon reign ended last month, and he came to Beijing knowing he would lose the No. 1 ranking to Rafael Nadal next week after 4 1/2 years on top.

"It was one of the goals of the season for me to do well here," Federer said. "This obviously is a big blow, because I expected more."

Federer said he may have made a mistake this year by playing too much and not practicing enough. But he blamed his latest defeat in large measure on Blake.

"I've played him on many occasions, but I think this was the best I've seen him," Federer said. "I'm happy for him. He's a good guy. I hope he can go all the way now."

The elimination of Federer means no rematch in Sunday's final against Nadal, who won in epic fashion when they met for the Wimbledon title. There will be no rematch of the Wimbledon final between the Williams sisters for a gold medal, either.

No. 4-seeded Serena struggled with her serve early against Dementieva, then staged a rally in the final set. Williams overcame two match points during an 18-point game to hold for 5-3.

But Dementieva held at love in the next game, sealing the victory when Williams pushed a volley wide. The loss came after the U.S. team had won 12 consecutive matches over the past three days.

"There's no reason I should have lost," Williams said. "I missed some key shots that I probably shouldn't have missed. That was that."

A wayward forehand and 12 double-faults doomed Venus Williams. The reigning Wimbledon champion sent a forehand long to lose serve and fall behind 6-5 in the second set, then had three more forehand errors in the final game.

When Li hit a service winner on match point, the crowd responded with the biggest roar of the tournament.

"She played the match of her life," Williams said. "Not much I could do about it. So it was definitely a surprise result for me."

The Williams sisters remained in contention for a doubles medal and were later to play a second-round match together. They won a gold in doubles in 2000 in Sydney, and Venus also won the gold there in singles.

Federer had been seeking his first medal after losing in the singles semifinals in Sydney and in the second round in Athens. He too remains in contention in the doubles competetion with Swiss partner Stanislas Wawrinka.

Federer played the first match on center court and seemed off his game from the start. His forehand -- once the sport's most feared -- was unreliable, and he repeatedly struggled to hold serve.

Blake earned the first break in the final game of the opening set. On set point, Federer left his feet for a spectacular backhand save that extended the rally, but with his next shot he floated an easy backhand into the net.

His shoulders sagging, he was broken again two games later and fell behind 3-0 in the second set.

Federer finally showed life by breaking back in the fifth game and holding the rest of the way to reach 6-all. But Blake played a flawless tiebreaker, while Federer made two unforced errors and popped up a volley.

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