Former Wimbledon champion Petra Kvitova has snapped Samantha Stosur’s seven-match winning streak to power into the quarter-finals of the Rogers Cup in Toronto.
Sixth-seeded Kvitova downed 12th-seeded Stosur 6-3 6-3 in the third-round showdown of two 2011 grand slam champions.
World No.1 Serena Williams eased into the quarter-finals in straight sets, while stomach problems sent Wimbledon champion Marion Bartoli crashing out on Thursday.
Two-time winner Williams avenged an earlier loss by her sister, Venus, to Kirsten Flipkens by routing the Belgian 6-0 6-3 in a third-round match at the $US2.4 million ($A2.65 million) tournament at the Rexall Centre stadium.
Flipkens, a surprise semi-finalist at Wimbledon, ousted Venus Williams 0-6 6-4 6-2 in the first round.
Williams, the reigning US Open and French Open champ, won this tournament in 2001 and 2011 and was the runner-up in 2000.
The American blasted 10 aces, had just one double fault and lost just one point on her second serve in the 56-minute match.
Williams moves on to the quarter-finals where she next faces Slovak Magdalena Rybarikova.
French star Bartoli retired when trailing Rybarikova 7-6 (7-5) 1-0.
Last week Rybarikova captured her second consecutive Washington crown.
Third-seeded Agnieszka Radwanska dispatched 14th-seeded American Sloane Stephens 6-1 7-6 (7-2) and fourth-seeded Chinese star Li Na rallied to beat former world No.1 Ana Ivanovic 3-6 6-1 7-6 (7-5).
Former French Open champ Li battled back from 5-2 down in the third set to beat Ivanovic.
“At the start of the second set, I began hitting the ball into the court more,” said Li.
“If I played a little bit soft, she was attacking a lot.”
Li said it became a battle of attrition so she just kept being aggressive.
“I was just trying to see if I could turn it around, and I’m happy I was able to. Sometimes you can’t play your best, but you just hang in there until the end,” said Li, who lost to Kvitova in last year’s Rogers Cup final.
Fifth-seeded Italian Sara Errani outlasted France’s Alize Cornet 7-5 7-6 (7-3). Errani will meet 2012 Wimbledon runner-up Radwanska in the round of the eight.
Romanian Sorana Cirstea surprised 15th-seeded Serb Jelena Jankovic 6-3 6-4.