"I'm disappointed to have to withdraw from the finals," Osaka said in a statement. "This is not how I wanted to end this tournament or my season but I look forward to getting healthy and hope to be back here next year."
Alternate Kiki Bertens will take Osaka's place against Barty. Osaka opened the tournament by beating Petra Kvitova 7-6 (1), 4-6, 6-4 in the Red Group. Barty also won her first match, beating Belinda Bencic 5-7, 6-1, 6-2.
Osaka said the injury surfaced in Beijing this month. She won tournaments in Osaka, Japan, and Beijing heading into the WTA Finals. "Actually I did this in the finals of Beijing," she said at a news conference. "I didn't serve for a while when I was in Japan. I just started serving the last two days. Felt better, came here, then I played my match.
"I felt it immediately (while playing Kvitova). When I woke up the day after, it was like throbbing and stuff. Yeah, didn't serve at all yesterday. Hit for like five minutes." Osaka's WTA Finals debut in Singapore last year also ended on a low note with a 0-3 record in round-robin matches. She retired with an injury against Bertens in her third match after losing the first set.
"This is the second time I had to withdraw from the Finals," Osaka said. "The last time I at least played, so a retirement instead of a withdrawal. "I thought I was playing well. I definitely wanted to win here."
As an alternate, Bertens is eligible to advance to the semifinals, which is determined by match wins.
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