The heavy rain had delayed the competition for almost four hours and then came back to hit the schedule again after midnight when three matches were still going on.

Federer became the largest seed casualty of the day as the Swiss world number one lost to American eighth seed James Blake in the men's singles quarterfinals.

Federer was playing below standard, slumping at 6-4, 7-6 (7-2) in just one hour and 30 minutes.

"I'm speechless. This is unbelievable. It's not very often you get to say you beat the No. 1 player in the world anytime, but to do it at the Olympics with USA on your chest, I'm so proud to be a part of this team," said Blake.

It was the American's first win over the Swiss. Federer entered the match with an 8-0 record in matches against his opponent and had only ever lost one set to Blake.

To a noisy home crowd, Chinese player Li Na produced another upset by edging past American seventh seed Venus Williams in women's singles quarterfinals.

The former world top 20 player came into the Olympics unseeded and had beaten Russian third seed Svetlana Kuznetsova in the first round of the event.

Li pulled back from 4-1 down to steal the first set 7-5 from the Sydney 2000 gold medalist, who served nine double faults in the opening set and was kept guessing by Li's variety of shots.

Li retained her momentum in the second set, going 3-1 up and breaking again in the 11th game to sneak ahead 6-5. On Li's second match point, Williams put a forehand service return into the net.

"It's still overwhelming. I can't believe I made it. It gave me more confidence to go further," said Li, who will in the semifinals play the winner of the match between sixth seed Dinara Safina and second seed Jelena Jankovic.

"Everyone knows she is a good player, the best in the world, and she won Wimbledon this year. At the beginning of the match she had a huge, big serve and I could not pick the ball, but I managed to tell myself play the ball back and try my best."

In an early match, Serena Williams, Venus' younger sister, was also knocked out from the quarterfinals after losing to Russian fifth seed Elena Dementieva.

This marks the second time this year that both Williams sisters fell in the same round on the same day at a big tournament. The two went out in the third round at the French Open.

Elsewhere, Spaniard Rafael Nadal stayed on course for the gold medal after breezing through Austrian Jurgen Melzer 6-0, 6-4 in one hour or so.

His reward will be a semifinal clash against Serbian third seed Novak Djokovic, who squeezed past Frenchman Gael Monfils 4-6, 6-1, 6-4.

The Athens 2004 bronze medalist Fernando Gonzalez also made it to the last four after beating Paul-Henri Mathieu from France.

He will take on American Blake in the other semifinals.

Russia are guaranteed a finalist in the women's singles and therefore at least one medal as Dementieva will in the semifinals play teammate Vera Zvonareva, who beat Sybille Bammer from Austria 6-3, 3-6, 6-3 in the quarterfinals.

"I'm very excited for the moment. I'm very excited about my semifinals here and I don't want to think about a medal. I just want to be focused on my next match and take it step by step," said Dementieva.

Zvonareva had been expected to play only in the doubles but replaced compatriot Maria Sharapova when she withdrew from the Olympics due to a shoulder injury.