American Jessica Pegula won 10 of the last 12 games to beat an error-prone Elena Rybakina and notch a 7-5 6-2 win in Group A at the WTA Finals in Cancun, Mexico, on Sunday.

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 at the prestigious season-ending tournament.

"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 Maria Sakkari of Greece, who was battling top seed Aryna Sabalenka in Sunday's Group A night match.

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

(Reporting by Rory Carroll in Los Angeles; Editing by Michael Perry)