Jessica Pegula upset world number one Aryna Sabalenka 6-4 6-3 to win her second group stage match at the WTA Finals in Cancun and advanced to the semi-finals 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.
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