From 1-all at the outset, Swiatek seized complete control of Monday night's decider in Cancun, Mexico, collecting the next 11 games with her high-quality baseline game, rarely making an unforced error and repeatedly pressuring Pegula into mistakes.
The 22-year-old from Poland went 5-0 at the tour's season-closing championship, winning all 10 sets she played and ceding a total of just 20 games. That's the fewest by the tournament's winner since 2003, when it returned to a round-robin format; the previous low in that span was the 34 games dropped by Justine Henin in 2007.
"Today was like a highlight of this mental (approach) — just being narrow with your head, only thinking about the right stuff," said Swiatek.
Swiatek is also the youngest WTA Finals champion since Petra Kvitova was 21 in 2011.
Swiatek extended her winning streak to 11 matches and improved to 68-11 with six trophies in 2023, including at the French Open in June for her fourth grand slam title.
This victory allows her to return to No.1, a spot Swiatek held from April 2022 until this September, when she relinquished it to Aryna Sabalenka. Swiatek defeated Sabalenka in a semi-final that began Saturday, was suspended by rain, then finished on Sunday.
Pegula hadn't dropped a set entering Monday and eliminated her doubles partner, US Open champion Coco Gauff, on Saturday. The 29-year-old American was bidding to become the oldest first-time WTA Finals champion and to pick up what would have been the most important trophy of her career.
After beating No.1 Sabalenka and No.4 Elena Rybakina in the group stage, then No. 3 Gauff, Pegula's matchup against No.2 Swiatek made the American the first woman to face each of the top four players in the world at one event since the start of the WTA rankings in 1975.
But Pegula's nine-match winning streak was stopped emphatically by Swiatek.
They played under a mostly blue sky and with far less wind than players were forced to contend with throughout the week. And, most importantly, there wasn't a drop of rain.
The singles and doubles finals both originally were scheduled to be contested Sunday, then were pushed back a day after a series of showers during the week.
Laura Siegemund of Germany and Vera Zvonareva of Russia won the doubles title by beating Nicole Melichar-Martinez of the US and Ellen Perez of Australia 6-4 6-4.
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