Kiki Bertens beat world number one Ashleigh Barty at the WTA Finals just hours after being added to the tournament as a replacement.
The Dutch world number 10 took the place of Naomi Osaka, who withdrew on Tuesday with a shoulder injury.
Bertens was trailing by a set and a break but Australian Barty collapsed to a 3-6 6-3 6-4 defeat in Shenzhen.
Barty lost seven games in a row from 3-3 in the second set after a series of errors and could not recover.
"It is a great atmosphere and it's the last tournament of the year so I am trying to give all the energy I have left in me," Bertens said after recording her first win over the French Open champion.