Defending champion Serena Williams lost her opening match at the China Open on Wednesday, falling to 127th-ranked Chinese wild card Sun Tiantian by 6-2, 7-6 (7).
It's the first time 9th-ranked Williams, winner of seven Grand Slams, has lost to anyone outside the top 100 in a main draw. It was Sun's first win over anyone in the top 40.
Williams hasn't reached a final since winning the Australian Open in February, and has been bothered by shoulder, ankle and knee problems.
"I would say the main reason for my loss today is my left knee," said Williams, who had a first-round bye.
"I just played terrible. Sun played very well and was ready for the match. I'm disappointed I couldn't perform better today," she said.
Sun startled the home crowd by jumping ahead 2-0 and easily taking the first set. In the error-filled match, Williams frequently sprayed balls long, hit drop shots short, and double-faulted. She was broken to trail 4-5 in the second set, broke Sun back in the next game, but Sun prevailed in the tiebreaker 9-7.
"Before the match I was just saying don't be nervous," said Sun, through to only her second career quarterfinals.
"I had an opportunity today and I grabbed it. I just hope I can keep improving. My goal for this year is to crack the top 100," she said.
Williams, whose injuries forced her to miss the French Open and didn't help in her earliest defeats at Wimbledon and the US Open since 1998, said she was keen to rest and recover.
"I definitely think I'm going to get well before I perform again, for sure," she said.
Sun's victory capped a great for China: wild card Zheng Jie upset No. 6-seeded Jelena Jankovic of Serbia-Montenegro 7-5, 4-6, 7-5 in a first-round match, and Peng Shuai beat Maria Diaz-Oliva of Argentina 3-6, 6-2, 7-6 (0) in a second-round encounter.
Seventh-seeded Tatiana Golovin of France also lost, to Japan's Shinobu Asagoe 6-3, 6-4.
In other first-round action, Israel's Shahar Peer breezed past Russia's Vera Douchevina 6-2, 6-0 and will next face another Russian -- world No. 1 Maria Sharapova, and Poland's Marta Domachowska cruised past Meghann Shaughnessy of the US 6-3, 6-3.
Germany's Anna-Lena Groenefeld defeated Japan's Ai Sugiyama 6-2, 6-3, and will meet Peng in the quarterfinals.
Venus Williams yesterday cruised to an easy 6-3, 6-1 win over Spain's Nuria Llagostera Vives, joining World No. 1 Maria Sharapova in the quarterfinals.
Sharapova overcame an unsteady second set to beat Israel's Shahar Peer 6-0, 5-7, 6-2 earlier yesterday to book her place in the round of eight.
Venus Williams, the No. 3 seed, dominated her match from the start, sprinting to a 4-0 lead in under 10 minutes. She later sealed the set with an ace.
The second set was more of the same, with Williams pounding her way to victory in less than an hour.
The top-seeded Sharapova was tested by Peer but persevered in their two-plus hour match.
"I focused in the third set and won it. Obviously I'll need to improve for my next rounds, but I'm pretty sure I'll get better," Sharapova said.
It was Sharapova's first match at the tournament, since she had a bye through the first round.
"The first match is always going to be tough because you don't know what to expect," she said.
In other action yesterday, Russia's Maria Kirilenko defeated Japan's Aiko Nakamura 6-1, 7-6 (3). Shinobu Asagoe, also of Japan, sailed past Jill Craybas of the US, 6-2, 6-4. Poland's Marta Domachowska beat China's Zheng Jie 6-3, 6-1.