Coco Gauff battled to a three-set victory over fellow American Jessica Pegula as she made a winning start to the WTA Finals.
Gauff, the 2024 champion, demonstrated resilience and superb athleticism to overcome 2023 runner-up Pegula 6-4 6-7 (5-7) 6-2 at Indian Wells in California.
Gauff - at 22, the youngest player to appear at five consecutive WTA Finals since Martina Hingis in 2000 - last competed at the China Open in early October.
"I'm really proud of myself for this one," she said.
"To play a tournament like this with no preparation is something I've never done in my career, but I'm just really happy to get through.
"It was a tough match. I was having a lot of fun."
Gauff has won only one title this year, when she beat Pegula in the Cincinnati Open final in August.
Pegula, 32, won titles in Dubai and Charleston and reached the semi-finals of the Australian Open and US Open.
She is only four wins behind Elena Rybakina's tour-leading 54 this season, despite playing only one match since the US Open ended in September.
"Jess is an incredible player. We always go back and forth," Gauff said.
"Today the key was just trying to be aggressive, but I also had to run a lot.
"My shoes are pretty torn up now. Some of my shoes are somewhere on the court there. So I'll have to change shoes tomorrow."
Next Gauff will face Wimbledon champion Linda Noskova, who beat Marta Kostyuk 6-2 6-4 in the opening match of the Finals. Pegula will take on Kostyuk.
Czech 21-year-old Noskova won all four of her break points against Ukrainian Kostyuk, who was given a late call-up after world number one Rybakina withdrew on Saturday because of illness.
The Finals features the top eight players this season and consists of two groups of four in a round-robin format. The top two from each group progress to the semi-finals, before the final on Sunday.
Against Pegula, Gauff recovered from three break points down to hold for a 5-4 lead in the first set before breaking Pegula in the following game.
Pegula won each of Gauff's first three service games in the second set as she raced into a 5-1 lead, but Gauff took the next three games.
Pegula, who wasted three set points when serving at 5-4, unleashed a yell of frustration and slammed her racquet into the ground after missing two break points in the following game.
Pegula survived match point on her own serve to force - then win - a tie-break.
But Gauff dominated the deciding set, comfortably winning the first three games before converting a third match point and letting out an emotional roar.