INDIAN WELLS, Calif. -- Coco Gauff rallied to win the final six games of the match and beat Marta Kostyuk 6-4, 7-5 in her round-robin group at the WTA Finals on Thursday.
Gauff's eventual fate in the season-ending championships depends on the outcome of fellow American Jessica Pegula's match against Wimbledon champion Linda Noskova later Thursday.
A victory by Pegula would knock Gauff, her former doubles partner, out of the $10 million event, with Pegula advancing to the semifinals. Gauff beat Pegula in three sets in their opening Group 1 match.
Noskova has already advanced to the semifinals.
Gauff overcame serving issues and visits from a trainer during the match.
Kostyuk raced to a 4-0 lead in in the second set, fighting off four break points to hold in the second game. Twice, Gauff double-faulted away games. She requested a trainer early in the set to work on her as she lay on her side while Kostyuk walked over to sit on the advertising board and chat with her team.
Kostyuk led 5-1 before Gauff's comeback began. She overcame three break points and hit a crosscourt forehand winner to trail 5-2. Gauff broke twice to tie the set at 5-5. Gauff held for a 6-5 lead and won the match on Kostyuk's netted forehand.
Gauff led 5-2 in the first set before Kostyuk broke to close to 5-4. Gauff broke back in the next game with a gutsy crosscourt backhand volley winner to take the set.
Kostyuk got into the Finals as the first alternate after top-ranked Elena Rybakina withdrew because of a viral illness. The 24-year-old Ukrainian went 0-3 in Group 1 play.