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.
Kasatkina managed only three winners as former world number one Sharapova overpowered her in a performance which showed her credentials before the US Open this month.
“I came on court having a lot of respect for my opponent because she’s up and coming but has already established herself,” world number 22 Sharapova said.
“I knew I had a tough match ahead of me. I was focused from the beginning and I finished it that way.”
British number one Johanna Konta plays Belarusian two-time Grand Slam winner Victoria Azarenka in the second round later on Wednesday.