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 Li Na, Wimbledon winner Petra Kvitova and third seed Vera Zvonareva all went out in round three.
Serena Williams, unseeded as she continues her comeback from injury, beat China's Zheng Jie 4-6 6-3 6-3.
Victoria Azarenka 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 Galina Voskoboeva's 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.
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 Sam Stosur.
"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.
"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 Andrea Petkovic of Germany.
Polish 13th seed Agnieszka Radwanska 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 Vinci followed up her win over Wozniacki with another impressive result as she beat former number one Ana Ivanovic 7-6 (7-3) 6-2.
Czech Republic's Lucie Safarova upset eight seed and 2010 French Open champion Francesca Schiavone 6-3 6-3.