Angelique Kerber's first match since she won Wimbledon ended in a 6-4 6-1 defeat by France's Alize Cornet at the Rogers Cup in Montreal.
World number four Kerber, 30, who beat Serena Williams to claim her third Grand Slam title, made 32 unforced errors in the second-round match.
Cornet, ranked 30 places below German Kerber, won in 85 minutes.
Maria Sharapova hit 25 winners as she thrashed fellow Russian Daria Kasatkina 6-0 6-2 to reach the third round.
The five-time Slam champion, 31, needed an hour 16 minutes to see off 21-year-old Kasatkina, who is considered one of the rising stars of the women's game and ranked 12th in the world.