World number two Jabeur kept her chances of WTA Finals success intact as she came from behind to defeat Pegula.
Having lost to Aryna Sabalenka in her opening match in Texas, US Open and Wimbledon runner-up Jabeur was staring down a group-stage exit from the tournament when she found herself 1-6 2-3 down halfway through the second set on Wednesday.
Yet the 28-year-old hit back to triumph 1-6, 6-3, 6-3 over her American opponent, the world number three who has now lost both of her matches in the Nancy Richey group.
Jabeur reeled off four straight games to take the momentum and, after fending off four break points to hold serve in the first game of the deciding set, did not look back, clinching a decisive break to nose 5-3 ahead.
A stray Pegula cross-court forehand wrapped up Jabeur's comeback win, and the Tunisian, who is aiming to become the first African player to reach the semi-finals at the season-ending tournament, can now turn focus to her final group game against Maria Sakkari, who she has faced three times previously, losing twice.
Jabeur's victory was her third against a top-three opponent, after beating Simona Halep in 2018 and Karolina Pliskova in 2020, while only world number one Iga Swiatek has won more matches this season (47).