Venus and Serena suffer defeats

Serena Williams will compete with her sister Venus in the doubles

Sisters Serena and Venus Williams saw their hopes for Olympic gold in the women's singles end with surprise quarter-final defeats in Beijing.

China's Li Na had the home crowd in raptures when she defeated seventh seed Venus 7-5 7-5, while Russia's Elena Dementieva upset Serena 3-6 6-4 6-3.

"I didn't serve well. It hasn't sunk in yet," said fourth seed Serena.

Meanwhile, Vera Zvonareva is also through to the last four after beating Austrian Sybille Bammer 3-6 6-3 6-3.

Venus seemed to wilt under the Centre Court floodlights as the partisan crowd inspired Li to victory.

The Wimbledon champion made a number uncharacteristic errors on her forehand and was guilty of 12 double faults on her serve.

606: DEBATE Who do you think will win Olympic gold?

The 26-year-old added: "Wow! You can't see this kind of crowd anywhere else. It helped me and pushed me a lot.

"This is a present I give to the Beijing Olympic Games but I have no time to celebrate. I have more matches to go."

Li, ranked 42 in the world, was 3-0 behind in the first set but broke back for 4-3, and again broke for 6-5 to hold her serve for the set.

Venus then slumped 3-0 behind in the second set only to fight back to lead 4-3. However, Li made the decisive break in the 11th game and showed great composure to serve for the match.

Earlier in the evening, Serena looked on course for victory when she powered through the opening set, but Dementieva soon got to grips with her opponent and began to dominate from the baseline.

Dementieva led 5-1 in the final set but had to hold off a determined fightback from the Wimbledon runner-up.

"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," said Dementieva.

The Williams sisters will now focus on winning the doubles tournament together, a title they won in Sydney eight years ago.