Aryna Sabalenka ended top-ranked Iga Swiatek's 15-match streak against top-10 opponents with a 6-2 2-6 6-1 victory that put her into the championship decider at the WTA Finals.
The stunning result put Sabalenka into the final against Caroline Garcia, who beat Maria Sakkari 6-3 6-2 and now has a chance to be just the second Frenchwoman to win the title at the season-ending event.
Swiatek, the French Open and US Open champion and runaway leader with eight tour victories, cruised through three round-robin victories, losing just 13 games to give her the longest winning run against top-10 opponents since Steffi Graf won 17 straight in 1987.
But it was suddenly brought to a halt when the 21-year-old from Poland lost the last five games against a player she had beaten in all four meetings this season.
"I just want to make sure that every time she plays against me, she knows that she really has to work hard to get a win," Sabalenka said. "Only because of this thinking, I was able to play at this amazing level tonight."
Swiatek fell behind one break in the final set with two wide forehands before another one put her down two breaks.
Just 24 hours after beating Daria Kasatkina in a tense 80-minute third set to secure the last spot in the semi-finals out of group play, Garcia needed just 74 minutes total for a career-best fourth victory over a top-five opponent this season.
Garcia never trailed, dominating the fifth-ranked Sakkari in winners (21-8) and aces (6-0).
Amelie Mauresmo (2005) is the only woman from France to have won the WTA Finals.
"I don't know," Garcia said when asked where she found the energy to dominate after the quick turnaround.
"Yesterday, I was a little bit tired, but it was nothing unusual after such a big match."
Garcia has advanced out of group play in both WTA Finals appearances. The 29-year-old lost in the semi-finals in the eight-player event five years ago.
"I guess I'm five years older, maybe five years wiser," said the sixth-ranked Garcia, who was No.74 about this time last year.
"You try to learn from everything. I've got a good team behind me, supporting me when I was a little bit doubting myself."