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.
"She had a few net cords and good shots on some of the important points at the start of the second set."
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.
She had won just four matches on clay before her meeting in Paris with Kudermetova, who was playing in only her second Grand Slam main draw.
"I hadn't really been able to play or practise for some weeks, then played my first points a couple of days ago and went from there basically," Wozniacki said.
"But my calf now feels good, so that's a positive. I have to try and stay positive and obviously it's not as easy to stay positive when things aren't going your way."
Wozniacki's exit comes after Czech sixth seed and Australian Open finalist Petra Kvitova withdrew on Monday with an arm injury, while Wimbledon champion Angelique Kerber suffered a shock 6-4 6-2 defeat to world number 81 Anastasia Potapova on Sunday.
Kudermetova will now face Kazakh world number 99 Zarina Diyas in the second round.
Elsewhere, Dutch fourth seed Kiki Bertens comfortably defeated French world number 66 Pauline Parmentier 6-3 6-4 on Suzanne-Lenglen.
And Australian Open quarter-finalist Ashleigh Barty, seeded eighth, beat American world number 72 Jessica Pegula 6-3 6-3.
Latvian 12th seed Anastasija Sevastova won 6-1 6-4 against Thailand's Luksika Kumkhum, but could face a tougher test in the second round.
She faces Mandy Minella, who caused an upset when she beat world number 13 Anastasia Pavlyuchenkova 6-4 6-2.
Meanwhile, German 18th seed Julia Gorges suffered a disappointing 7-5 6-1 defeat by Estonian world number 88 Kaia Kanepi, a quarter-finalist at the US Open in 2017.