Naomi Osaka has withdrawn from the WTA Finals because of a right shoulder injury ahead of her scheduled round-robin match against top-ranked Ash Barty.
Alternate Kiki Bertens will take Osaka's place against Barty later Tuesday.
"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."
The two-time Grand Slam title winner opened with a 7-6 (1), 4-6, 6-4 win over Petra Kvitova in the Red Group.
Barty opened the round-robin stage with a 5-7, 6-1, 6-2 win over Belinda Bencic.
As an alternate, Bertens is eligible to advance to the semifinals, which is determined by match wins.