Pegula recovered from a set-point battle and saved a match point before completing a 7-6(2), 4-6, 7-5 victory over the Czech player in a contest lasting approximately two hours and 45 minutes on Thursday.
The American made an impressive contribution with her serve, recording a career-best 16 aces as she fought her way through one of the tournament's most demanding encounters.
Noskova, who had already secured qualification for the semi-finals, responded strongly after losing the opening set, taking the second 6-4 to force a decider.
The deciding set produced further drama. Pegula faced a match point at 4-5 but managed to save it before holding serve, breaking Noskova and eventually closing out the contest.
Gauff eliminated despite victory
Pegula's triumph also ended Coco Gauff's hopes of defending her WTA Finals title.
Earlier, Gauff had defeated Marta Kostyuk 6-4, 7-5, recovering from a 1-5 deficit in the second set to win six consecutive games.
However, the result was not enough to secure her progression, as all three players finished the round-robin stage with two victories and one defeat.
Noskova topped the group with a 5-2 sets record, while Pegula finished second with 5-3. Gauff's 4-3 record left her in third place and outside the qualifying positions.
Pegula prepares for Andreeva semi-final
Pegula will face Russian teenager Mirra Andreeva in Saturday's semi-final as she attempts to reach the WTA Finals championship match.
The upcoming meeting has attracted additional attention following Pegula's private comments about Andreeva during a practice session earlier in the tournament, which were captured on a live broadcast.
The American later apologised for the remarks and acknowledged Andreeva's qualities as a player.
Meanwhile, Noskova will face the runner-up from the other group, with Aryna Sabalenka and Elina Svitolina competing for the remaining qualification positions.
Pegula's victory has kept her title hopes alive, while Gauff's campaign ends despite finishing the group stage with the same win-loss record as the two players who progressed.