Before Monday, Osaka had won 14 of her previous 16 matches to be the in-form player coming into the season-finale.
"I'm just really happy to get through and play a good competitive match", Stephens said on court after the match.
In a match between the last two U.S. Open champions, Sloane Stephens came out on top.
"That was something I'm not really proud of". She reached her second career Grand Slam final at this year's French Open, and won the Miami title.
After a solid hold from Stephens, Osaka was struggling to contain her emotions as she found the net or went long with her increasingly erratic backhand and the American secured her third break of the match to move a game away from taking the set. When she failed to capitalize on four break points in the fourth game of the second set, she made a visible shrugging gesture with both hands. But Osaka appeared to be mentally worn out and only managed to hold serve in the third game of the final set.
"I think, in a way, that you can only get better every match you play".
"I took my five-minutes of being really sad", Osaka said.
Bertens, only in the eight-player field when world number one Simona Halep withdrew through injury, had been swept aside 6-1 in the first set and was 2-1 down in the second to Kerber when she called on Raemon Sluiter for a tactical debate.
"I'll do everything to win here, and that's what we're gonna do for the next match, and hopefully I feel good then". "We decided I should go a little bit more for my shots and be more aggressive".
Stephens, who later said the surface at the Singapore venue was providing "weird" bounce, joins Czech player Karolína Pliskova and Ukraine's Elina Svitolina as victors in their opening matches of the WTA Finals. "I think when I have that type of feeling, I outwardly express more than keeping everything inside".