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 teh Greek raced through with loss of just one point.

The top seed dug deep to take the second set on another tiebreak 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.