Sloane Stephens secured a 7-5 4-6 6-1 victory over Japan's Naomi Osaka in a battle between the last two US Open champions at the WTA Finals.
Both women are making their debuts at the season-ending event in Singapore, and it was the American who opened Red Group action on Monday with a win built on solid defence from the baseline and a devastating forehand when she attacked.
"I just competed really well and never gave up," Stephens said.
"I knew I would have to play some really good tennis to beat her, stayed tough and tried to stay as positive as I could and took my opportunities when they presented themselves."
After trading early breaks, Stephens broke again in the seventh game but the 21-year-old Japanese hit back to level the contest at 4-4, thanks to some sublime stroke-making.
After a solid hold from Stephens, Osaka suddenly started finding the net or went long with her increasingly erratic backhand. And the American secured her third break to move a game away before sealing the opener on her third set point, when the world No.4 sent another backhand into the net.
Osaka changed her tactics at the start of the second set, drawing Stephens way from her baseline comfort zone to hold far more comfortably.
Stephens regained her composure but Osaka battled back from 15-40 to hold in the fifth game and came out on top of three breaks of serve to send the contest into a decider when the American double-faulted on set point.
The world No.6 broke immediately at the start of the third set and after Osaka wasted a chance to level things up when she declined to challenge a call that would have been ruled in her favour, Stephens took command of the match.
A pair of easy holds and an another break took her to 5-1, and the 2017 US Open winner sealed the contest on her second match point when an anguished Osaka double-faulted for the fourth time in the match.
"We played a really tough and competitive match so its always an honour to share the court with somebody that good," Stephens added.
Red Group play continues on Monday when Wimbledon champion Angelique Kerber takes on Kiki Bertens of the Netherlands, who was the last player to make it into the eight-woman tournament when world number one Simona Halep withdrew through injury.