"I woke up this morning and couldn't really move my thumb," Osaka told reporters. "I tried to practise and grip my racket but I couldn't, and I kept feeling this pain when I tried to move my hand in different directions.
"Right now I'm... 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.
"I didn't feel anything yesterday. That's why I'm kind of confused right now because I literally woke up in the morning and couldn't move my thumb. So I was like, maybe I slept on it and maybe it will go away, but it didn't."
An abdominal injury had forced Osaka to pull out ahead of her semi-final clash in Stuttgart last month before she was beaten by Belinda Bencic in the Madrid Open quarter-finals.
"I would describe (my clay season) as rocky," Osaka added.
"But 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 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."
Following her U.S. Open and Australian triumphs, Osaka is chasing a third straight Grand Slam trophy at the French Open which begins on May 26.
(Reporting by Shrivathsa Sridhar, editing by Pritha Sarkar)
