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.

One crumb of comfort for Osaka is that she can still advance to the semi-finals if she wins at least one of her remaining pool matches against Angelique Kerber and Kiki Bertens.

"Yeah, I think it's really cool that you can keep going even after you lose. I think that this tournament is really special in that way, because there is no other tournament during the year that does a round-robin format," she said.

Bertens battled back from a set down to defeat top seed Kerber 1-6 6-3 6-4 later on Monday.

Bertens, who entered the tournament as the lowest-ranked eighth seed after world No.1 Simona Halep pulled out with a back injury, hit 33 winners to help her succeed in two hours.

It wasn't all plain sailing for Bertens, though, as Germany's Kerber raced through the first set, and 12 double-faults and three breaks of her own serve in the final set made her look vulnerable.

But the 26-year-old Dutchwoman relentlessly attacked Kerber's serve as the reigning Wimbledon champion crumbled to surrender five straight breaks of her service from 4-3 down in set two to concede defeat.

Bertens is the first woman from the Netherlands to break into the world's top 10, at number nine, since Brenda Schultz-McCarthy in 1996.