Wimbledon champion Angelique Kerber suffered a shock defeat to Alize Cornet in the second round of the Rogers Cup in Montreal.
Kerber, playing her first match since she won her third Grand Slam title at the All England club last month, was second best as her unseeded opponent won 6-4 6-1.
Cornet will meet Ashleigh Barty in the third round after victory over the world No 4 - her second victory over the German on hard courts.
There was no such surprise defeat for Maria Sharapova as the former world No 1 made light work of her 12th-seeded compatriot Daria Kasatkina.
Sharapova used her aggressive baseline game to good effect as she overwhelmed her 21-year-old opponent in an emphatic 6-0 6-2 victory to book a meeting with Caroline Garcia.
Fifth seed Elina Svitolina also reached the third round, but the fifth seed was pushed hard by San Jose champion Mihaela Buzarnescu, before the Romanian was forced to retire with an injury.
Svitolina, the defending champion, was leading 6-3 6-7 (5-7) 4-3 when Buzarnescu suffered a nasty fall, appearing to injure her ankle.
Another seed to fall was former world No 1 Karolina Pliskova as the Czech ninth seed was beaten 6-2 6-2 by Kiki Bertens.
Third seed Sloane Stephens overcame Canadian wild card Francoise Abanda 6-0 6-2.
British No 1 Johanna Konta is due in second-round action against Belarussian wild card Victoria Azarenka later on Wednesday.
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