Improved serving from both players ensured the opener went to a tiebreak and after Stephens staked another 3-0 lead, the American edged ahead on her third set point when Bertens sent a forehand into the tramlines.
With the duo's battle taking almost two-and-a-half hours, the second match of the night started at around 10.30pm, and it was another three-set, two-hour-plus battle before Kiki Bertens - a first-time singles participant and a last-minute replacement for the injured Simona Halep - stunned top-seeded Angelique Kerber 1-6, 6-3, 6-4.
The key moment of the match was the second game in the final set Stephens after the break with the score 0:40, he defended his serve and never allowed Osaka to return to the game.
US Open champion Naomi Osaka will use her WTA Finals defeat to Sloane Stephens as a learning tool, acknowledging the ruthless nature of the Singapore tournament. She was rewarded with a break and closed out the first set, much to the racquet slinging frustration of Naomi Osaka.
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.
"I think she played really well and I think she deserved to win the match".
It was the result Osaka needed after she fell to No. 1 seed Angelique Kerber in a three-set classic to put her on the brink of elimination.
World number six Stephens has moved into pole position after her second straight victory.
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"Yeah, I think it's really cool that you can keep going even after you lose".
But the Japanese insists there is no need to regret the reverse as she settles in among the world's best players.
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.
Bertens qualified just days before the tournament after world number one Simona Halep pulled out due to injury, which bumped up Kerber to top billing.
A pair of easy holds and an another break took her to 5-1, and the 2017 US Open victor sealed the contest on her second match point when an anguished Osaka double-faulted for the fourth time in the match. "I just made her play an extra ball".
"At the beginning of the second, it was just like hopefully I can get a few more games".
She said: "Obviously Kerber, I have played some really great matches against her, so looking forward to that".