Naomi Osaka has withdrawn from the WTA Finals in Shenzhen because of a right shoulder injury.
The world number three is replaced in the end-of-season event by reserve Kiki Bertens, who will take on top seed Ashleigh Barty in the second round of group matches later on Tuesday.
Osaka won her opening match on Sunday, defeating Petra Kvitova in three sets in a compelling replay of their Australian Open final.
The 22-year-old said: "I'm disappointed to have to withdraw from the Finals. It has been a great event in Shenzhen, and it's the biggest WTA event of the year.
"This is not how I wanted to end this tournament or my season. I look forward to getting healthy and hope to be back here in Shenzhen next year."
Dutchwoman Bertens made her debut at the WTA Finals last year, reaching the semi-finals before losing to eventual champion Elina Svitolina.
She reached a career-high ranking of four in May and currently sits 10th having lost to Aryna Sabalenka in the final of the WTA Elite Trophy last weekend.