Former Australian Open champion Caroline Wozniacki says she “ran out of steam” in a surprise first-round defeat by Russian world number 68 Veronika Kudermetova at the French Open.
The Danish 13th seed lost 0-6 6-3 6-3 despite winning the first seven games and said her opponent had “got lucky”.
“It’s definitely frustrating,” the 28-year-old former world number one said.
“It hasn’t been a great year for me so far, so I’m going to work hard and try and turn it around.”
Wozniacki, who wore strapping on her left calf, won 62% of the total points in the opening set but crumbled in the second and was broken three times.
She went 3-0 behind in the decider before losing a first-round match in Paris for only the third time since 2007.
“I think she got very lucky at the start of the second set and took advantage of the opportunities she got,” Wozniacki added.
“I ran out of steam in the end and made some unforced errors that I don’t usually do.
The Dane, a two-time quarter-finalist at Roland Garros, withdrew from her first-round match against American Danielle Collins at the Italian Open earlier this month because of a calf injury.
It was the second successive tournament Wozniacki retired early from after she pulled out of her opening match at the Madrid Open with a back injury.
Meanwhile, British number one Johanna Konta earned her first victory in the French Open main draw with a straight-set win over Germany’s Antonia Lottner.
Konta, seeded 26th, won 6-4 6-4 against the world number 147.
The former Wimbledon semi-finalist is the first Briton through at Roland Garros, with men’s number one Kyle Edmund meeting France’s Jeremy Chardy.
Compatriots Cameron Norrie and Dan Evans play their opening matches on Tuesday.