A year ago in Fort Worth, Texas, Jessica Pegula made her WTA Finals debut and lost all three of her singles matches.
This year at the same event in Cancun, Mexico, Pegula became the first player to qualify for the semifinals.
The fifth-seeded Buffalo, N.Y., native upset top-seeded Aryna Sabalenka of Belarus 6-4, 6-3 on Tuesday to improve to 2-0 in the Bacalar Group. Pegula is guaranteed to finish in the top two of the group at the end of the round-robin opening round.
Sabalenka fell to 1-1.
In the day's other match, fourth-seeded Elena Rybakina of Kazakhstan defeated eighth-seeded Maria Sakkari of Greece 6-0, 6-7 (4), 7-6 (2) in a matchup of players who had dropped their opener in the Bacalar Group.
Two groups of four players are competing in separate round robins in the tournament's first phase. The two top players in each group advance to the semifinals on Saturday, with the Saturday winners competing for the championship on Sunday.
The victory over the No. 1-ranked player was the third for Pegula this year, following wins over Poland's Iga Swiatek in Sydney and Montreal.
"I feel more comfortable playing (top opponents) this year," Pegula said. "I feel more confident. I don't think it feels like such a big deal. Last year when I made top five, I think I psyched myself out a little bit — not a lot. You have a couple thoughts in your head — do I really belong at that level?
"I was always good at beating those below me, but now you have to beat the one or two before you. I think you put a bigger emphasis on that. And I think this year, I've not really cared. I've had more confidence in those matches and that's why I've got more top wins in big moments. I feel that's where I've improved a little bit."
Pegula took advantage of sloppy play from Sabalenka, who committed 29 unforced errors and amassed just 17 winners. Pegula had 14 winners and 12 unforced errors.
The Chetumal Group returns to action on Wednesday. Third-seeded Coco Gauff of the United States will oppose Swiatek, the second seed, in a matchup of players who won their opener. Sixth-seeded Ons Jabeur on Tunisia and seventh-seeded Czech Marketa Vondrousova, who both dropped their first match, will meet in the day's other contest.
—Field Level Media