SHENZHEN, China (AP) World No. 1 Ashleigh Barty was upset by late substitute Kiki Bertens 3-6, 6-3, 6-4 at the WTA Finals on Tuesday.
Bertens arrived at the elite tournament as an alternate, and has suddenly jumped into title contention with the other seven players in the draw. She took the place of Naomi Osaka, who withdrew from the finals on Tuesday with a right shoulder injury ahead of her second round-robin match.
With little notice she'd be playing, Bertens earned her first career victory over Barty in five matches played.
Bertens came close to beating Barty in the Beijing semifinals this month, losing in a third-set tiebreaker.
Bertens is 1-0 and Barty 1-1 in the Red Group standings.
Last year, Bertens reached the semifinals in her WTA Finals debut, bowing to defending champion Elina Svitolina.
Bertens struggled against Barty to find her form in the first set, but settled in the second to even the set score. The Dutchwoman broke serve on a fourth break point in the opening game of the third, and raced 4-0 ahead.
Despite the loss, Barty officially secured the year-end No. 1 ranking. She's the first Australian woman to earn that distinction.
Only Karolina Pliskova and Osaka had an opportunity to prevent Barty from finishing the season as the year-end No. 1. They both had to win the title here with a perfect record in the round-robin.
Pliskova lost to Elena Svitolina in her opening round-robin match in the Purple Group on Monday.