It was a much-anticipated showdown between the past two US Open champions, who were making their WTA Finals debuts.
After trading early breaks, the match settled down as the pair enjoyed more success on serve, until Stephens, content to adopt a patient approach from the baseline, broke again in the seventh game when Osaka made a slew of unforced errors.
"I just tried my very best from the first point in the third set and it didn't go my way immediately, but I just kept battling", Stephens said after the match.
Stephens, who is without Kamau Murray this week due to a long-standing obligation, was pleased with the way she regrouped after having dropped the second set.
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.
Osaka lifted her intensity in the second set and rediscovered the form that has propelled her to a career-high world ranking of four.
Stephens - French Open finalist this season - mixed her trademark solid defence and counterpunch strategy to frustrate her inconsistent Japanese opponent.
"So I think that the fact that you get another chance to learn from your mistakes and apply it in the matches going on is something that's really good".
Osaka's serve deserted her in the final set as she was broken three times.
The second match was a slow burner, with the crowd taking time to warm to the contest, as Stephens raced into a 3-0 lead before Bertens dragged herself back thanks to some clever changes of pace and direction.
"But definitely can relate on being exhausted and needing to regroup and play - you know, I think she took the next week off".
The game got off to a sluggish start with both players struggling to hold serve and few winners being struck before Stephens made her move, aggressively rattling Osaka with deep groundstrokes and pinpointing the lines.
"Yeah, I think it's really cool that you can keep going even after you lose".
The second set did not disappoint with a competitive start before the break point chances came out to play. Osaka saved two set points at 5-6, but hit an unforced error off her forehand on the third. "I think there was a lot of stress during the match, so maybe not too good for the body".
She said: "Obviously Kerber, I have played some really great matches against her, so looking forward to that".
"Then I called him (Sluiter).in the first set".