American Stephens, the world number six, raced into a 3-0 lead in the opening set and the 25-year-old went on to wrap it up in 46 minutes.
Stephens broke back immediately and served out to take the set 6-4 and level the match.
Bertens failed to capitalize on four set points ahead of the second set tiebreaker, but pushed the match into a decisive third set with a return victor on her next set point. I know I played great tennis this week and I'm very happy to share this moment with you guys. "It means a lot to me", Svitolina said on court after the match.
"It was just getting worse and worse every time I played".
"I have done qualifying for my first final here, beat some fantastic players, played four really great matches".
"It will be very tough and it's the last match of the season so I'm going to give my best and leave everything on court". And mostly just maybe coming in a little bit more, taking time away from shorter alls, making sure she steps in on those and then finding her way to the net once or twice, like she's done yesterday with good success. It will be determined in a pair of Sloane Stephens - Caroline Plskova.
Sloane Stephens has battled back from dropping the first set to love to reach the final at the WTA Finals.More news: Report details how furious Aaron Rodgers was with Ty Montgomery
She lost her serve at love in the ninth game and then chose to stop.
The match started tensely, with Kerber unable to convert seven break points in Stephens´ first three service games, and the missed opportunities proved costly.
Stephens will play the sixth seed Elina Svitolina in the showdown after the 24-year-old Ukrainian took more than two and a half hours to see off the eighth seeded Dutchwoman Kiki Bertens.
Naomi Osaka crashed out of the WTA Finals when a tight hamstring consigned the U.S. Open champion to a third straight defeat in her tournament debut on Friday, but the rising Japanese star was still keen to reflect on what has been a brilliant campaign. "I think it helped me a lot with the confidence".
It ended the tournament's five-year run in Singapore with the season-finale moving to Shenzhen.
Svitolina has looked on a mission at the WTA Finals in an effort to make amends for a lacklustre early exit previous year.
But she did not relinquish her lead from then on, out-rallying Stephens to win the set and force a decider.