Jessica Pegula upset world No. 1 Aryna Sabalenka 6-4, 6-3 to win her second group stage match at the WTA Finals in Cancun on Tuesday and advanced to the semifinals after Elena Rybakina edged Maria Sakkari 6-0, 6-7(4-7), 7-6(7-2) in the evening match.
Pegula, who came into the contest with a dismal 1-4 record against the hard-hitting Sabalenka, managed to absorb her powerful serve to extend rallies and thwart the Belarusian's comeback attempts.
Sabalenka, who has said she did not feel safe playing on the newly constructed Estadio Paradisus, never looked totally comfortable against Pegula, committing 29 unforced errors on a sunny day in the coastal Mexican city.
The top seed appeared on the comeback march in the first set when, trailing 5-2, she pounded a thunderous forehand winner to break Pegula and backed that up with a service hold to pull within a game of the American.
But Pegula kept her cool and captured the first set after a tense rally ended with a backhand into the net by Sabalenka on set point.
Pegula raced out to a 4-0 lead in the second set, but Sabalenka once again rallied, breaking serve to cut Pegula's lead to 5-3.
It took seven match points in the marathon final game of the match for Pegula to finally deliver the knockout punch, which came when Sabalenka shanked a backhand to deliver Pegula her fifth straight win over a top-10 opponent.
After the match, Pegula pushed back on the notion that her game is lacking in power compared to the likes of Sabalenka.
"I hit hard," she told Tennis Channel.
"People don't seem to think I hit hard, like commentators. Seriously, I don't know why they say that. Maybe because it looks like I'm not? I hit flatter, I don't hit as heavy as maybe as Aryna, but I hit hard."
Pegula's second group stage win in a row ensured a place in the semi-finals later in the evening, as fourth seed Elena Rybakina shook off a late comeback attempt from Sakkari to get her first win in the competition.
Rybakina routed her Greek opponent 6-0 in the first set, but Sakkari fought back, taking the subsequent sets into tie-breakers despite Rybakina's commanding start in the third set that had her 4-2 up.
Sakkari broke Rybakina's serve for the first time and got the set to 6-6, but the latter remained calm to clinically clinch the tie-breaker and won the hard-fought clash that lasted two hours and 26 minutes.
"I felt at the beginning, for Maria, maybe it took her some time to get used to my speed," Rybakina said in a post-match interview about the one-sided first set.
"As we all know, the surface is not the greatest. We're struggling, all of us."
The Kazakh world No. 4 will take on Sabalenka on Thursday in a battle to reach the semis, while Pegula, already through to the semis, will face Sakkari in their final group stage match.
