Maria Sharapova was among the latest leading names to make an early exit at the Rogers Cup in Toronto on Thursday.
Fifth seed Sharapova, French Open champion
, Wimbledon winner
and third seed
all went out in round three.
, unseeded as she continues her comeback from injury, beat China's Zheng Jie 4-6 6-3 6-3.
is the highest-ranked player remaining after top seed Caroline Wozniacki's second-round exit.
Wozniacki, the world number one,
lost to Italy Roberta Vinci on Wednesday.
Sharapova lost 6-3 7-5 to become Kazakhstani qualifier
biggest scalp of the tournament so far.
The 26-year-old, ranked 135 in the world, beat Flavia Pennetta and Marion Bartoli en route to the last eight.
ROGERS CUP QUARTER-FINALS
- Roberta Vinci v Sam Stosur
- Agnieska Radwanska v Andrea Petkovic
- Galina Voskoboeva v Victoria Azarenka
- Lucie Safarova v Serena Williams
Sharapova admitted that she was short of match practice after taking time off to recover from a hectic schedule around the French Open and Wimbledon.
"All of us give ourselves a little bit of a vacation after that because we are on the road and travelling for back-to-back Grand Slams," she said.
"After that it takes everyone a little bit of time to get the rust off and to get going."
China's Li was playing her first match of the week after a bye and a walkover victory and was comprehensively beaten 6-2 6-4 by 10th seed
"It's always tough after a break to come back for the first match, because I had six or seven weeks that I didn't play a tournament," agreed sixth seed Li.
COUNTDOWN TO US OPEN
- 15-21 August - Cincinnati
- 21-27 August - New Haven Open
- 22-27 August - Texas Tennis Open
- 29 August -11 September - US Open
"At the beginning of the match I didn't even know what I should do on the court, not like during the clay-court season."
On a windy day in Toronto, Kvitova went down 6-1 6-2 to 11th seed
Polish 13th seed
repeated last week's victory over Zvonareva in last weekend's Carlsbad final, coming through 6-4 7-6 (7-4) this time.
Azarenka defeated Maria Jose Martinez Sanchez 6-1 6-2 and Italy's
followed up her win over Wozniacki with another impressive result as she beat former number one Ana Ivanovic 7-6 (7-3) 6-2.
upset eight seed and 2010 French Open champion Francesca Schiavone 6-3 6-3.