Serena Williams [Images] suffered one of the worst defeats of her career as she was humbled 6-2, 7-6 by local girl Sun Tiantian at the China Open on Wednesday.

The defending champion went down after a flurry of embarrassing errors against the Chinese wildcard, ranked 118 places below her at 127 in the world.

The seven-time Grand Slam winner saved four match points before Olympic [Images] gold medallist Sun converted her fifth chance with a forehand pass to win the tiebreak 9-7.

Fourth seed Williams looked close to tears as she served in the tiebreak but she refused to make excuses for her worst loss since joining the WTA Tour full-time in 1997.

"I played terrible," said the world number nine, who has battled knee and ankle injuries this year. "I never expected such a tough match. She hit some great shots.

"I'm still struggling a bit with my left knee but I'm disappointed not to have performed better."

Williams lost her first professional match as a 14-year-old 6-1, 6-1 to 149th-ranked Annie Miller in qualifying in Quebec City in November 1995. She did not play on the WTA Tour in 1996.

Sun, who won Olympic doubles gold with Li Ting in Athens last year, became the second Chinese player to reach the last eight in Beijing [Images].

"I never expected to win," said Sun. "It's definitely a breakthrough but it's not a miracle. I played the way I always play. It will give me a big confidence boost."

CHINA CHARGE

China's number one Peng Shuai beat Argentine Mariana Diaz-Oliva 3-6, 6-2, 7-6 to reach her fourth WTA quarter-final this year.

The world number 33 battled nerves to storm through the tiebreak 7-0 and will now play Germany's [Images] number nine seed Anna-Lena Groenefeld, who ousted Japan's [Images] Ai Sugiyama 6-2, 6-3.

Peng's compatriot Zheng Jie produced an upset with a 7-5, 4-6, 7-5 win over sixth seed Jelena Jankovic in a first round match on a successful day for the home players.

Serena's sister Venus, seeded three, plays Spain's Nuria Llagostera Vives in her second round match on Thursday, having only arrived in Beijing from Seoul on Tuesday.

The Williams sisters and top seed Maria Sharapova [Images] received first-round byes.

Second seed Lindsay Davenport [Images] withdrew with injury to simplify Wimbledon [Images] champion Venus's route to the final.

New world number one Sharapova faces Shahar Peer in her opening match on Thursday after the Israeli's 6-2, 6-0 win over another Russian, Vera Douchevina.

Frenchwoman Tatiana Golovin [Images] was another seed to fall on Wednesday, the number seven losing 6-3, 6-4 to Japan's Shinobu Asagoe in a first round match.

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