Naomi Osaka admitted her clay-court form ahead of the French Open has been "rocky" after she withdrew from the Internazionali d'Italia on Friday due to injury.
The defending US Open and Australian Open champion pulled out of her quarter-final clash with Kiki Bertens in Rome after waking up with pain and swelling at the base of her right thumb.
It was the second time in three tournaments that the 21-year-old had withdrawn due to injury, with an abdominal problem ending her run at the Stuttgart Open.
Osaka, who lost in the last eight of the Madrid Open this month, will be the top seed and world number one at the French Open, but she conceded her injury problems have been frustrating as she bids to make it three grand slam titles in a row.
"I would describe [the clay season] as rocky," she told reporters on Friday.
"I can't necessarily say it's been ups and downs because if I think about it, it's definitely been going up.
"Every match that I've played, I've learned a lot. I've tried to take what I've learned into the next match and I think I've done that well.
"But the injury part is very annoying. I feel like the ab thing could have been helped, but this one I don't think I could have helped because I don't know what caused it. I don't know why I have it.
"I feel there are preventable injuries and ones where you have to just take it how it is."
Osaka is not the only one to have suffered injury problems this week.
Serena Williams was unable to face sister Venus because of a recurrence of a knee injury, Caroline Wozniacki retired from her first-round match because of a calf complaint and Simona Halep, the defending French Open champion, needed treatment on a leg injury during her second-round loss to Marketa Vondrousova.
Osaka suffered her problem just nine days out from the French Open beginning and she expressed her disappointment at being unable to face the in-form Bertens, who beat Halep in the Madrid Open final last weekend.
"I woke up this morning and couldn't really move my thumb," Osaka explained.
"I tried to practice and grip my racquet, but I couldn't, and I kept feeling this pain when I tried to move my hand in different directions.
"Right now, I'm really not mad but in between sad and disappointed because I really wanted to play my match today. For me, it was sort of a test to play against Kiki because she's playing really well, and I wanted to see how well I could do today."