One day after claiming the world No 1 ranking, Serena Williams tumbled out of the China Open in Beijing with a 6-4, 3-6, 7-6 (5) loss to Russia's Nadia Petrova yesterday.

The American fought back from 0-5 to 4-5 in the tiebreak of the decisive third set but a controversial replayed line call stopped her from saving a second match point in a row.

Post-match, the 28-year-old said she had learnt from the painful lesson at last month's US Open, where she threatened to push a ball into a line judge's throat after a foot-fault call.

"I don't' know what really happened. The line judge called it out but we kept playing I'm still confused," Williams said.

"I'm just doing the best manners I can on the court. I'm let me think of a word smart, I've learned things from the past. Smart people never make the same mistakes they did in the past."

She also admitted Petrova played a better match than she did.

"I think she was reigning the whole match. I broke her one game in the second set but that was to make sure it was to go to the third set," she said of an opponent she led 6-1 in their previous encounters.

"I think she played her best match ever against me," Williams said.

The second seed's loss makes fourth seed Elena Dementieva of Russia the highest-ranked player left in the women's draw after top seed Dinara Safina fell to Chinese wild card Zhang Shuai in the second round and Venus Williams lost to unseeded Anastasia Pavlyuchenkova of Russia in her opening match.

Petrova's next challenge will be local Peng Shuai.

"I'm ready for the match. She plays very well here and this is her home court. But I will just focus on what I can do best and the result will follow," the 27-year-old two-time Grand Slam semifinalist said.

"I think my confidence and experience will help me through the match."

China's world No 53, Peng, cleared her path to the quarterfinals by upsetting defending champion Serbian Jelena Jankovic and Russian Maria Sharapova, the winner of last week's Pan Pacific Open.

The 23-year-old also advanced to the semis of the women's doubles yesterday with Hsieh Su-Wei of Chinese Taipei. They defeated Pavlyuchenkova and Belgian Yanina Wickmayer.

Along with facing a tough opponent, Peng, a two-time semifinalist at the China Open, is also burdened with mounting expectations from the home crowd as Zhang Shuai, the only other Chinese player left in the singles draw, was stopped by 14th seed Marion Bartoli of France in the third round yesterday. Bartoli won 6-1, 6-4.

Zhang, 20, attributed her loss to weak serves, just as her compatriot Zheng Jie did after being beaten by Svetlana Kuznetsova of Russia in the first round.

The world No 226 had four double faults and no aces in the 92-minute match while Bartoli fired in four aces.

"My delivery of service was not consistent enough and that made me unable to hit as hard as I can. I will try to improve it (serving) in the coming winter training," Zhang said.

The Frenchwoman will face seventh seed Vera Zvonareva of Russia, who downed 10th seed Flavia Pennetta of Italy 6-3, 5-7, 7-5 in a 152-minute match.

In another women's match, Spaniard Maria Jose Martinez Sanchez, who defeated US Open finalist Caroline Wozniacki of Denmark in the first round, failed to advance further as she lost to Agnieszka Radwanska of Poland 4-6, 6-4, 6-2.

The Pole will compete for a place in the semifinals against Dementieva.