British number one Heather Watson was knocked out in the second round of the Rogers Cup as she lost 6-4 6-3 to Andrea Petkovic.
German Petkovic punished the 23-year-old's faltering serve to win in an hour and 24 minutes in Toronto.
Defeat meant Watson missed out on a rematch with Serena Williams, whom she came close to beating at Wimbledon.
Wimbledon finalist and eighth seed Garbine Muguruza lost 7-5 6-1 to Ukrainian qualifier Lesia Tsurenko.
Romanian second seed Simona Halep reached the third round by beating former world number one Jelena Jankovic 6-3 6-4.
However, third seed and former Wimbledon champion Petra Kvitova lost 6-2 6-3 to two-time Australian Open champion Victoria Azarenka from Belarus.
Fourth seed Caroline Wozniacki was beaten 7-5 7-5 by Belinda Bencic of Switzerland and seventh seed Lucie Safarova lost 4-6 7-5 7-5 to Russia's Daria Gavrilova.