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.
Later, Swiss Belinda Bencic kept alive her hopes of reaching the semi-finals by battling past Petra Kvitova.
The world number seven stormed to the first set before being pegged back but came through a tight deciding set to win 6-1 3-6 6-4.
Bencic's victory means all four players can still finish in the top two in Red Group and progress to the last four from the round-robin stage.
Bertens will only play two matches because of her late entry but can still advance. She meets Bencic on Thursday.
Barty would have qualified with a match to spare if she had beaten Bertens but will get another opportunity when she takes on Czech Kvitova.
Kvitova, who has lost both matches so far, could still progress if she beats the Australian.
Bertens wins as Barty falters
Bertens had been in China this week in case of an injury to one of the eight players and had to step in to play Barty less than four hours after Osaka announced her withdrawal shortly before 07:00 GMT.
The 27-year-old, who made her debut at the WTA Finals in 2018 and reached the semi-finals before losing to eventual champion Elina Svitolina, started poorly, only holding serve once in four attempts in the first set.
She then twice trailed by a break in the second set - at 2-1 and 3-2 - before Barty's level dropped dramatically with number of errors.
Bertens won seven games in a row to clinch the second set and move 4-0 ahead in the third, and, despite a late rally from Barty, Bertens closed out the match on serve at the first opportunity.
"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.
"In the beginning I wasn't playing as well and had to feel the court and got better and better as I tried to play aggressively and it turned out pretty well."
The WTA Finals event features the world's top eight players competing in two round-robin groups of four, with the winners and runners-up advancing to the semi-finals.
Ukraine's Svitolina is in the Purple Group alongside world number two Karolina Pliskova of the Czech Republic, Romania's Simona Halep and Canada's Bianca Andreescu.