INDIAN WELLS, California, Nov 12 - Coco Gauff rallied to beat Marta Kostyuk 6-4 7-5 at the WTA Finals on Thursday and now needs fellow American Jessica Pegula to lose in order to reach the semi-finals of the season-ending championships.

After sealing the first set with a stab volley that sent fans leaping to their feet under sunny California desert skies, Gauff rallied from 1-5 down in the second, winning six games in a row to keep her hopes of a second WTA Finals title alive.

"I was fully prepared to play a third set there," said Gauff, who needed treatment on her hip during the second set.

Whether twice major champion Gauff advances to the semis or is eliminated now hinges on her friend and frequent doubles partner Pegula, who takes on Wimbledon champion Linda Noskova later on Thursday.

Pegula will advance if she beats Noskova, who has booked her spot in the semi-finals by winning the group.

Kostyuk, who won the Madrid title and reached the French Open semi-finals, was already eliminated coming into her match against Gauff having lost to Noskova and Pegula.

Kostyuk, who joined the eight-player tournament as an alternate after world number one Elena Rybakina withdrew due to illness, failed to win a set in her three matches. REUTERS