Late entry Kiki Bertens stunned world No 1 Ashleigh Barty with a comeback three-set victory to shake-up the season-ending WTA Finals in Shenzhen on Tuesday.
All four players can still qualify from Red Group after Belinda Bencic's three-set victory over Petra Kvitova in the later match.
Bertens looked in deep trouble at a set and break down, but turned the tide with ruthless returning and aggressive play at the net to overwhelm a deflated Barty, who could have sealed a spot in the semi-finals with victory.
The world No 10 won 3-6, 6-3, 6-4 in two hours and nine minutes in a see-saw match featuring 12 breaks of serve.
"Of course you're always ready to play, but I did not see it coming today," Bertens said of her late call-up. "In the beginning I was struggling. As the match continued, I felt better and better."
Bertens was a late replacement for world No 3 Naomi Osaka, who earlier in the day withdrew from the round-robin tournament due to a shoulder injury.
She appeared nervous under the bright lights dropping her first three service games and Barty, who has locked up the year-end No.1 ranking, pounced to run away with the first set.
Barty then endured her own service woes against an increasingly aggressive Bertens, who showcased a soft touch at the net.
A slumping Barty had her serve broken five times through the second and third sets as Bertens stormed to her first victory over the Australian.
Barty lamented letting slip her early lead. "I wasn't able to execute as cleanly when I was up in the score line," she said.
Bertens, who made the semi-finals of last year's edition, continued her strong form after making the finals in Zhuhai last week and the semi-finals of the China Open earlier in the month, when she lost a three-set thriller to Barty.
In the later match, Bencic fought back from a second set spiral to beat Petra Kvitova 6-3, 1-6, 6-4 in one hour and 47 minutes. It kept her tournament hopes alive after she had lost her opening match to Barty.
"I was proud that I was able to block away her big shots," Bencic said. "I was happy that at the end I was able to keep calm."
Kvitova's serve went off the boil as she fell away badly in the first set to lose five straight games. A double fault was a fitting way for the Czech to hand Bencic the first set.
But the match flipped in the second set with Kvitova, the 2011 WTA Finals champion, rediscovering her lethal serve to force a decider.
The twists continued in the third set with Bencic breaking in the third game as a flustered Kvitova slammed her racquet on the ground. The Swiss failed to capitalise in the next game but grabbed the decisive break in the ninth before confidently closing it out on serve.
Winless Kvitova plays Barty on Thursday and Bencic meets Bertens to determine the winners from Red Group.