British number one Johanna Konta was beaten 3-6 7-6 (7-5) 6-2 by Karolina Muchova in the first round of the Cincinnati Open.
Konta, 30, won the first set in an assured start before rain interrupted the second set in Ohio.
After the resumption, Czech Muchova, 24, claimed the second set in a close tie-break.
The world number 23 took the deciding set and will face former US Open champion Bianca Andreescu in round two.
In the men’s tournament in Cincinnati, British number two Cameron Norrie could not convert a set’s lead to victory as American John Isner fought back to win 4-6 7-6 (7-5) 6-4.
Andy Murray, who beat Richard Gasquet in his opening match, will play ninth seed Hubert Hurkacz in the Wednesday’s second round after the Pole defeated Alejandro Davidovich Fokina of Spain.
Konta has had a troubled season, with no back-to-back wins and long injury breaks before she won the Nottingham Open this summer, her first title since winning the Miami Open in 2017.
But her plans to use that momentum at Wimbledon were crushed after she was identified as a close contact when a member of her team tested positive for Covid-19.
She had to then withdraw from the Tokyo Olympics as she would have little time to prepare after recovering from the virus herself.
Konta looked to be returning to form at the Canadian Open but a knee injury cut short her campaign.
In a second-tier tournament in Chicago, 18-year-old Briton Emma Raducanu beat top-seeded Belgian Alison van Uytvanck 7-6 (9-7) 6-3 to make the second round.
Raducanu, who reached the fourth round at Wimbledon earlier this year in only her second professional tournament, made up 124 ranking spots to overcome the world number 60.
She will play 20-year-old Clara Burel of France next.