Top seed Aryna Sabalenka crashed out of the WTA Finals in Guadalajara on Monday, losing a gruelling three-setter against Greek fourth seed Maria Sakkari.
Sakkari set up a semi-final against Estonia's Anett Kontaveit after outlasting Sabalenka 7-6 (7/1), 6-7 (6/8), 6-3 in 2hr 47min.
"It was a rollercoaster match from both of us," Sakkari said.
"It was just a matter of who took the chances. I think at the end I just played with my heart and fought well, just turned things around."
Sabalenka was left ruing a failure to capitalise on key moments and an eye-watering 19 double faults that undermined her service game.
The 23-year-old Belarussian squandered the chance to secure the first set, allowing Sakkari to recover from 5-3 down to force a tiebreak which the Greek raced through with loss of just one point.
The top seed dug deep to take the second set on another tie-break after again letting Sakkari back from 5-3 down.
But a weary Sabalenka wilted in the decisive set of the marathon match, serving 10 double faults.
"I had a lot of opportunities and I didn't use them," Sabalenka said.
"The whole match I was really struggling with my serve. On some moment I found it, and on some moment I couldn't do anything. Just crazy how bad I was serving."
Sakkari believed her physical conditioning helped get her over the line.
"I always say that fitness is one of my strengths," said Sakkari. "It's something that makes a lot of players kind of like intimidated because of that.
"They know that I can last long. They know that once I step on the court, I can stay there for three, four, five hours, as long as I have to."
Sakkari, 26, is chasing only her second WTA title after a win at the Morocco Open in 2019.
The right-hander is competing at the end-of-season women's championships in Mexico after a 2021 campaign that included semi-final appearances at the French Open and US Open.
The other semi-final will see an all-Spanish showdown between Paula Badosa and Garbine Muguruza.
Badosa, who was already assured of a place in the semi-finals, wrapped up her group stage with a 5-7, 4-6 loss to already-eliminated Iga Swiatek of Poland.
UNBELIEVABLE??
— wta (@WTA) November 16, 2021
???? @mariasakkari fights her way into the semifinals after an incredible match. She defeats the world No.2 Sabalenka 7-6(1), 6-7(6), 6-3?? #AKRONWTAFinals pic.twitter.com/byzs9mWErs
Round Robin
Maria Sakkari (GRE x4) bt Aryna Sabalenka (BLR x1) 7-6 (7/1), 6-7 (6/8), 6-3
Iga Swiatek (POL x5) bt Paula Badosa 7-5, 6-4 (ESP x2)