Former world number one Caroline Wozniacki retired from her first-round match at the Italian Open because of a leg injury.
Wozniacki, 28, lost the opening set 7-6 (7-5) against American Danielle Collins before she had to stop.
It is the second straight tournament at which the Dane has withdrawn early on.
She trailed 3-0 against Alize Cornet in the opening round of the Madrid Open earlier this month and had to pull out because of a back injury.
Wozniacki is ranked 12th in the world and will be hoping to be fit for the French Open, which starts on 26 May.
Collins, 25, will now play two-time Grand Slam winner Garbine Muguruza of Spain in the second round on Wednesday.
“Obviously, I feel for Caroline, what she’s going through with her injuries,” said Collins.
“It’s never fun to go out there and play against somebody that’s hurt, and dealing with that kind of pain. I was happy with the way I played, but it’s just not a fun situation.”