Bertens, meanwhile, continues to thrive after showing marked improvement on all courts as opposed to her favourite clay surface, backing up her opening win over Kerber and three-set loss to Stephens with another solid display in Singapore.
"It was such a tough battle today and I'm very happy I could win in the end. I was making a lot of shots in the court, but she was just reading everything that I was putting back", Stephens added.
"It was just getting worse and worse every time I played".
In truth, Stephens wasn't really tested against Kerber and got through the match playing in second or third gear.
"But I was staying very positive and I think that's what made the difference this week. Then I won one game and you know the rest". "I think mental toughness is what I really had to show". Stephens brought it back to 3-2, but Svitolina ran the table from there to put the match away 6-2 with the help of a flurry of errors from the American.
Stephens, the 2017 US Open champion, will face Karolina Pliskova of the Czech Republic, while Bertens will face Ukraine's Elina Svitolina.
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WTA Finals Singapore live stream, TV channel details plus previews of the semi-finals and final, exclusive videos and odds as the tournament reaches a conclusion this weekend.
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.
And the world number seven continued her dominance into the final set, breaking three times once more to secure the title.
But it was enough for Bertens to seal a final four spot in her debut at the season finale, having defeated top seed Kerber earlier in the round-robin phase.
"I was clear with what I had to do on court", she said. It was Pliskova's first win over her compatriot in four matches between the two.
"Unfortunately, this is the a year ago that Singapore is hosting this wonderful tournament". The Ukrainian had two opportunities to break Stephens' serve in the next game, but the latter saved the first break point with a forehand victor down the line and the second with a backhand cross-court victor.
Svitolina has looked on a mission at the WTA Finals in an effort to make amends for a lacklustre early exit past year.