World number three Pegula needed nearly three hours to dethrone Swiatek as the duo rode a rollercoaster of breaks and re-breaks before the 31-year-old American took a 5-4 lead in the final set after Swiatek saved three break points but double-faulted on a fourth.Pegula quickly closed it out, firing a service winner on match point to reach Sunday's championship match in the US Open tune-up.Pegula will face either unseeded Czech Sara Bejlek or fourth-ranked Coco Gauff in Sunday's final.Bejlek, 20, went into their Saturday night semi-final seeking another big upset, having knocked off world number one Aryna Sabalenka in the fourth round.France's Arthur Fils charged into the men's final, dispatching French Open runner-up Flavio Cobolli 6-3, 6-4 in a match delayed for 45 minutes by rain and lightning.Swiatek and Pegula then waited out another rain delay before their match, which was also briefly interrupted two games in.The first five games of what evolved into an epic went against the server as Pegula and Swiatek struggled to adjust to the conditions.Swiatek blinked first as Pegula held at love for a 4-2 lead, but Swiatek won the next three games before Pegula broke again for a 6-5 lead -- taking the set when Swiatek smacked a volley into the net on set point.They had exchanged two breaks in the second when Swiatek grabbed a lifeline and broke in the final game to square the match.Swiatek will drop from fifth to eighth in the world after failing to defend her title, but it's been a productive August for the former world number one who claimed her first title of the year in Toronto this month.In men's action 22-year-old Fils booked his first ever Masters 1000 final, where he'll face the winner of the all-American semi-final between Brandon Nakashima and Frances Tiafoe.Fils's 68-minute win over seventh-seeded Cobolli made him the youngest French Masters finalist since a 20-year-old Richard Gasquet in Toronto two decades ago.Fils admitted he had been unsettled by the weather delay.