Kasatkina, who needed a lengthy medical timeout in the third set, booked her place in Sunday’s final after prevailing 6-4, 4-6, 7-6 (7/5) in 2hr 45min against home hope Pegula.
The Russian fourth seed finished the semi-final with her right leg heavily strapped after a gruelling scrap against her opponent.
Kasatkina looked to be in trouble in the final set tie-break after Pegula raced into a 3-1 lead.
But the 26-year-old –- who won the Charleston crown in 2017 -– showed great courage to rally back and take control.
She moved to double match point at 6-4 in the tie-break after Pegula smacked a return into the net.
Pegula saved the first match point after Kasatkina hit a forehand long, but the Russian made no mistake on the next point, whipping a superb crosscourt forehand beyond her opponent to seal victory.
The defeat marked a disappointing end to the tournament for Pegula, who had advanced to the last four after an incredible Houdini act in the quarter-finals when she saved four match points before squeezing past Victoria Azarenka.
“One day you’re winning match from match points, next day, you’re going to lose,” Kasatkina said after Saturday’s win. “I call it ‘tennis drama’ — it has to be like this.”
Kasatkina will play another American, the in-form Danielle Collins, in Sunday’s final.
Collins, brimming with confidence following her recent Miami Open victory, took down Greek third seed Maria Sakkari 6-3, 6-3 to continue her recent fine run of form in what is the final season of her career.
The world number 22 is now on a 12-match unbeaten streak and Saturday’s win means she has won 21 matches this year — tied for third best on tour alongside Iga Swiatek and Elena Rybakina.
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