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 on Tuesday and will advance to semi-finals if Elena Rybakina beats Maria Sakkari 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 feeling 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 at heavy as maybe as Aryna, but I hit hard."

Pegula will get a chance to show off her easy power once again when she faces Sakkari in her final group stage match on Thursday. REUTERS