"I'm just really proud of my fight today", she continued.
With both trying to put pressure on each other as they rallied, Svitolina was the steadier of the two in the second set.
The Ukrainian raced to a 3-0 lead in the third set, but Stephens managed to secure a break in the fifth game.
From there Stephens found her footing and seventh-seeded Pliskova began to derail.
Svitolina held another tight service game to stay alive in the opener, but Stephens sealed the set when her opponent could only find the net with a backhand. I was, like, I'm going to lose 0-0. Bertens lost just once, to Sloane Stephens, but even then, made the American work very hard for the win. Nothing's working. But then I won one game. "A lot of the top girls who have up-and-down results, a win here and then they lose there, we all kind of deal with the same thing", she told reporters.
Svitolina could rise three spots to world number four on the rankings after her triumph and she has now won nine straight finals.
"I think I have nothing to prove anymore to anyone and it's definitely a good statement for myself", Svitolina said.
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Fast forward 12 months and she was not coming in with quite as much form as a year ago (albeit not much in it) and her participation was out of her hands. With the Dutchwoman threatening to break back, it was Svitolina's determination and ability to track down every ball that forced Bertens to go aim for the line as she just missed.
Bertens played too risky a brand of tennis, posting 41 winners to 12 for Svitolina, but also making 63 to 36 unforced errors. The eight seed responded with a forehand victor and pushing Svitolina around the court to get the break back. "It's going to be very, very tough - but it will be my last match of the season, so I will leave everything out on the court", the 24-year-old world No. 7 added.
This is the first year since 2011 that Serena Williams didn't win a major title, and she's 37 years old now.
Bertens battled back as she broke in the opening game of the second set but Svitolina took the tenth game to make it 5-5 before taking the set into a tie-break and Bertens levelled the match.
The WTA Finals ends its five-year run in Singapore this season and moves to Shenzen, China for a 10-year run starting in 2019. It will be determined in a pair of Sloane Stephens - Caroline Plskova.