Bertens, who was a doubles finalist past year in Singapore, snapped a run of service breaks with a vital hold from 0-40 in the eighth game of the final set before a forehand victor earned the world No 9 a tour-leading 11th victory over a top 10 player this season.
Where it initially looked like Osaka had settled the quickest after the brace of breaks at the start with convincing looking holds to love, Stephens slowly began to make the adjustment to the court, hitting a consistently heavy ball and generating more chances towards the end of the set.
Osaka - who shocked Serena Williams to claim this year's grand slam in NY - 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.
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.
A calm Sloane Stephens picked her shots to perfection to overcome Japan's Naomi Osaka in a battle between the last two U.S. Open champions at the WTA Finals on Monday before top seed Angelique Kerber was upset by Kiki Bertens.
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Osaka, 21, rallied and closed out a 55-minute second set 6-4.
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 just competed really well and never gave up", said Stephens.
Osaka, however, too often found nothing but net in the third set, dropping to her knees when she netted a return that handed Stephens a 3-1 lead.
"I feel like there is a rhythm that I need to get into that I haven't really been in touch with for the past few weeks", she said.
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Bertens only qualified just days before the tournament after world number one Simona Halep withdrew due to injury.
A demoralized Stephens had no answer and her slump continued into the deciding set, where she was broken in the second game. "I just made her play an extra ball". With both players serving poorly, Kerber's next break eventually proved decisive.
"Usually after I lose, I have very specific things that I think would have changed the match". "This match was sort of a continuation of my last match in a way. that I didn't serve well".
However, Osaka fought back to earn two break points and levelled the match at 5-5 with a brilliant forehand victor. "Naomi, she's a great player and playing with a lot of confidence right now".
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