Aryna Sabalenka played "unbelievable tennis" to shock world No 1 Iga Swiatek 6-2, 2-6, 6-1 on Sunday to reach the WTA Finals final, where she will meet Caroline Garcia after the Frenchwoman cruised past Maria Sakkari in Fort Worth, Texas.
Swiatek had not dropped a set at the season-ending tournament but was subdued by the powerful forehand of the Belarusian, who sealed the massive upset with one of her 12 aces before pumping her fist in triumph.
"The only thing I had in my mind was just make her work. And that's it," Sabalenka said. "It's only because of that I was able to show unbelievable tennis tonight and get this win."
After trading breaks early in the first set, Sabalenka, who lost to the Pole in the US Open semi-finals in September, easily converted a couple of chances as Swiatek stumbled with a series of uncharacteristic mistakes.
Swiatek returned with new resolve in the second set, sweeping the first four games with the momentum seemingly on her side.
But world No 7 Sabalenka bounced back in the third set, dropping only two of her first-serve points without facing a break point to claim her third career win over a number one opponent.
"I just said that I have to stay aggressive and I have to stay in the moment, that's it," she said.
The win set up a rematch with a red-hot Garcia, who beat her en route to winning the Cincinnati title.
The Frenchwoman eased past Sakkari 6-3, 6-2 to reach her first WTA Finals championship match, sending down 19 winners and six aces to close out the tie in 75 minutes.
Garcia had needed three sets to beat Sakkari in Cincinnati three months ago but had a much smoother time of it on Sunday, breaking the Greek to love late in the first set before carrying the first four games of the second.
"My game style today, she didn’t like it," Garcia said.
A clumsy double-fault handed Sakkari a much-needed break point chance in the sixth game but Garcia snuffed out the danger with an ace and maintained the momentum.
Garcia thrust her hands into the air as Sakkari whacked the ball into the net on the second match point, bouncing around the court in celebration having reached the biggest final of her career.
The WTA Finals championship match is set for Monday.