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.

Roger Federer of Switzerland returns to James Blake of the US during their mens's singles quarterfinal tennis match at the Beijing 2008 Olympic Games, August 14, 2008. [Agencies]

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.