Aryna Sabalenka and Jessica Pegula off to winning starts in Cancun WTA Finals

Two of Sakkari's three wins in nine meetings with Sabalenka had come in previous editions of the season finale but the Greek world number nine was no match this time for her opponent, who moved to the top of Group A alongside American Jessica Pegula.

Sabalenka converted five out of nine breakpoint chances and won 11 games in a row during the clash to finish in 74 minutes.

"She's an amazing player. She's always fighting for every ball," Sabalenka said.

"I knew no matter what the score is that she would fight for it. That's what makes her a really tough opponent.

"You know against Maria, any time she can come back. You give her a little bit and she'll take that and keep fighting. I just tried to stay in the moment."

Australian Open champion Sabalenka overtook four-times major champion Iga Swiatek and captured the top ranking after the US Open and will retain the number one spot if she goes undefeated in round-robin play or makes the final.

Earlier, Pegula won 10 of the last 12 games to get past an error-prone Elena Rybakina and notch a 7-5 6-2 win.

The big-serving Rybakina looked poised to capture the first set when she stepped to the service line leading 5-3 in the opener but Pegula raised her return game for a crucial break.

Pegula used that momentum to close out the set by winning four straight games and her clean ball striking overwhelmed the Kazakhstani, who tallied 35 unforced errors in the match.

"I feel like she just made a couple careless errors and I was able to take advantage of it at the right time and ride that momentum all the way through the second set," Pegula told Tennis Channel after securing the win.

"I thought I was returning really well, reading her serve a little bit more, and was able to play some good tennis."

Pegula, who is seeded fifth, will on Tuesday face eighth seed Sakkari of Greece.

The October 29 to November 5 Finals features the top eight singles players and doubles teams in the world.