British number one Johanna Konta lost in the first round of the Rogers Cup in Toronto, beaten 6-3 6-2 by Ukrainian teenager Dayana Yastremska.
The 28-year-old world number 14 had beaten the 19-year-old in straight sets in their only previous meeting, on the Eastbourne grass in June.
But having taken a 2-1 lead Konta lost the first set in 37 minutes.
She hit only five winners to 24 from the powerful world number 33, who won the match in one hour 21 minutes.
It was the first match since Wimbledon for both players, Konta having reached the quarter-finals before losing to Czech Barbora Strycova, while Yastremska went out to Shuai Zhang of China in the last 16.