"I came out here a little bit nervous and I wasn't quite feeling the ball," said the fifth-seeded Stephens, who is making her debut at the tournament this year. "I just tried to stay in it. I was very proud of my effort today."
Both Svitolina and Stephens posted 3-0 round-robin records before winning their semifinals.
Stephens, the 2017 U.S. Open champion, let out a huge yell on breaking her Czech opponent's serve in the third game of the second set after trailing 6-0, 2-0.
From there Stephens found her footing and seventh-seeded Pliskova began to derail.
After Stephens had lost the first eight games, the American said she was thinking "I made it all the way to the semifinal, and I'm going to lose? ... I was, like, I'm going to lose 0-0. It's like I'm trying so hard. Nothing's working. But then I won one game. Then you know the rest."
Stephens reached her second Grand Slam final at the French Open in June. She has won one title at Miami this season.
Pliskova was only a point away from a 3-0 lead in the second set.
"I think set and a half I was playing very well," said Pliskova.
Last year Svitolina became the first Ukrainian woman to play singles at the WTA Finals. This year Svitolina became the first woman from her country to reach the season-ending final after beating Kiki Bertens 7-5, 6-7 (5), 6-4.
"It feels amazing and it was such a tough battle today," said Svitolina, who pumped her fists and then blew kisses to the crowd after winning.
"I think physically in the end it was just about running and chasing every ball."
Bertens played too risky a brand of tennis, posting 41 winners to 12 for Svitolina, but also making 63 to 36 unforced errors.
"There's still lots of work ahead," said sixth-seeded Svitolina. "It's the last match of the season, definitely now, and I'm going to give my best and leave everything on court."
Svitolina is chasing her fourth title this season, having already won at Brisbane, Dubai and Rome.
Svitolina qualified for a second consecutive WTA Finals this year only because her Dutch opponent on Saturday failed to reach the Moscow semifinals. The eighth-seeded Bertens eventually qualified for the eight-player field when No. 1 Simona Halep withdrew from the tournament with a back injury.
Bertens saved one set point on her serve in the 10th game of the first set, but couldn't repeat that trick in the 12th game, double-faulting at 15-40 to surrender the set.
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 winner on her next set point.
Serving for her place in the final, Svitolina saved two break points before shutting Bertens down.
This year is the first time since the round-robin format was reintroduced in 2003 that none of the top four seeded players — No. 1 Angelique Kerber, No. 2 Caroline Wozniacki, No. 3 Osaka and No. 4 Petra Kvitova — reached the semifinals.
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