BEIJING - Serena Williams was upset in the second round of the China Open on Wednesday, losing to unheralded Sun Tiantian 6-2, 7-6 (7).

Sun, ranked 127th in the world and fourth among Chinese players, took advantage of numerous errors by Williams, who said she was bothered by an ailing left knee.

"I just played terrible," Williams said. "Sun played very well and was ready for the match. I'm disappointed I couldn't perform better today."

Williams, No. 9 in the world rankings, won last year's event and is very popular in China. However, the crowd cheered as Sun pulled off a surprise victory - the first of her career against a player ranked in the top 40.

　　北京时间9月21日晚，雅典奥运会女双冠军、中国选手孙甜甜在中网女单第二轮比赛中，6-2/7-6(7)爆冷击败卫冕冠军、种子选手小威廉姆斯，晋级中网女单八强。