French Open champion Li Na played like a "junior" while Vera Zvonareva, Maria Sharapova and Petra Kvitova were all also sent packing after rusty performances in the third round at the Toronto Cup Thursday.
Serena Williams, unseeded following an 11-month layoff that ended in June, emerged as a tournament favorite with a 4-6 6-3 6-3 win over Chinese qualifier Zheng Jie that puts her into the quarter-final of a draw without 12 of its 16 seeded players.
Li, who had a first-round bye and a walkover in the second round after her opponent withdrew, looked out of sorts during a 6-2 6-4 loss to number 10 seed Samantha Stosur played during a power outage that kept the main scoreboard dark for an hour.
"At the beginning of the match I was like -- I didn't even know what I should do on the court, not like during the clay court season," Li, seeded sixth, told reporters. "I know exactly if I hit the ball next here, I come back, what I should do. But I was feeling like a junior on the court."
Li, playing in her first match since a shock exit in the second round of Wimbledon, had five double faults in another day of windy conditions, struggled with the high bounce of Stosur's serve and was unable to break her opponent.
Sharapova fell 6-3 7-5 to 26-year-old Kazakhstani qualifier Galina Voskoboeva, who has become an upset specialist this week after having beaten three top 25 players en route to a berth in the quarter-final.
Fifth-seed Sharapova said time off after a packed schedule surrounding the French Open and Wimbledon was likely to have played a role in a shaky performance where she committed five double faults and had her serve broken five times.
"All of us give ourselves a little bit of a vacation after that because we are on the road and traveling for back-to-back grand slams," she told reporters. "After that it takes everyone a little bit of time to get the rust off and to get going."
With the win, world number 135 Voskoboeva advanced to the quarter-finals against Victoria Azarenka, who at number four is the highest seed remaining.
Seventh seed Kvitova, who also had a first-round bye and was playing in just her second match since winning her maiden grand slam in July, appeared unable to shake off the rust that had plagued her during the second round.
"Well yesterday was a bad match for me after the one month (off), and I didn't play well," said Kvitova. "But today it was windy again like it was yesterday, and my game wasn't so good. I did many, many mistakes on the forehand."
Williams overcame a slow start against Zheng to book her spot in the quarter-final, where she will play Czech Lucie Safarova, a 6-3 6-3 winner over Italy's Francesca Schiavone, who became the fifth seed to fall Thursday.
In other action, third seed Zvonareva fell 6-4 7-6 to Agnieszka Radwanska, Azarenka breezed by Spanish qualifier Maria Jose Martinez Sanchez 6-1 6-2 and an inspired Roberta Vinci carried momentum from her shock defeat of top seed Caroline Wozniacki into her last 16 match to beat Ana Ivanovic 7-6 6-2.
In Friday's other quarter-final matches, Vinci faces Stosur and Radwanska plays 11th seed Petkovic.