In a clash of two debutants at the WTA Finals, the American world number six overcame the US Open champion to record a win in the Red Group.
Sloane Stephens marked her first WTA Finals outing with a 7-5 4-6 6-1 victory over fellow debutant Naomi Osaka on Monday.
A topsy-turvy battle between the most recent winners of the US Open saw the 2017 champion at Flushing Meadows eventually prevail after two hours and 24 minutes on court.
Osaka - who shocked Serena Williams to claim this year's grand slam in New York - fought back from a set down only to fade badly down the stretch, allowing Stephens to pick up the first points in the Red Group.
The tone was set for the match when both players struggled on serve in the early stages, Osaka losing her opening game before hitting back immediately to draw level.
She did the same again after netting a backhand to be broken for a second time to trail 4-3, though Stephens helped her opponent out with a pair of double faults when 40-30 ahead.
However, crucially the American held after breaking for a third time, allowing her to take the opener after 51 minutes.
The unpredictable nature of the contest continued in the second set, Osaka failing to take four break points in the fourth game before having to battle back from 15-40 down on her own serve in the next.
That hard-fought hold triggered a three-game mini run for the third seed, though the pair exchanged further breaks before Stephens - who often relied on her superb defensive capabilities to launch effective counter strikes against the aggressive Osaka - gifted the set to her opponent by sending down a double fault.
The fifth seed's disappointment did not last long, though, as she quickly seized control in a decider that saw her Japanese rival at one stage drop to her knees in a clear sign of frustration.
With a slice of good fortune - Osaka opted not to challenge an overruled service call that would have made it 1-1 having lost serve in the previous game - Stephens raced through the third set to make a winning start in Singapore.