Kiki Bertens took advantage of her unexpected chance at the WTA Finals by coming from a set down to defeat world number one Ashleigh Barty.
The Dutchwoman arrived in Shenzhen as an alternate but Naomi Osaka's withdrawal on Tuesday because of a shoulder injury gave her an opportunity.
It looked like being a short-lived campaign to reach the knock-out stages when Barty led by a set and 3-2 but the Australian went badly off the boil, losing eight games in a row to trail 4-0 in the decider.
She mounted a comeback but Bertens played a fine final service game to win 3-6 6-3 6-4.
Osaka won her opening match on Sunday, defeating Petra Kvitova in three sets in a compelling replay of their Australian Open final, but a right shoulder problem suffered earlier this month curtailed her campaign.
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.
She will take on Belinda Bencic in her second group match on Thursday, where victory would give her a great chance of making the last four again.
Barty defeated Bencic in her first match and will play Kvitova next.
All four women in the group still have a chance of qualifying for the semi-finals after Bencic claimed her first victory at the tournament, beating Kvitova 6-3 1-6 6-4.
Defeat for the young Swiss would have eliminated her from contention but Bencic bounced back strongly after losing a one-sided second set.