Swiatek will take on Elena Rybakina for the title at the US Open tune-up event after the Australian Open champion beat Coco Gauff 5-7 6-2 6-2 in Wednesday's (Thursday AEST) second semi-final.
Swiatek, who won one of her six grand slam titles at Flushing Meadows in 2022, secured the rain-delayed first set in Toronto before Svitolina turned the tables and broke the Pole twice in a one-sided second set.
Svitolina broke to grab a 3-2 lead in the decider but former world No.1 Swiatek reeled off the final four games to claim victory.
"After that second set I knew I had to do everything I could to get my rhythm and my game back," she told Tennis Channel.
"It's not easy to do that during a match, to keep your calm and your composure and follow the plan. I'm happy in the third set I did that and it paid off in the end. I didn't doubt myself."
Swiatek has had an inconsistent season, failing to advance beyond the quarters at a slam and replacing her coach after a surprise second-round defeat by compatriot Magda Linette in Miami.
Gauff, meantime, edged a tight opening set but the American's game fell apart after her serve was broken in the second, as Rybakina won five-straight games to level the contest.
Kazakhstan's Rybakina, the 2022 Wimbledon winner, broke twice in quick succession to take command of the third set and fought off a couple of break points to serve out the win.
The only previous meeting between Gauff and Rybakina also came in Toronto in 2022, which the American won in three sets.
"Last time we played, it was such a battle and even in practice, it's always a close match," world No.2 Rybakina, who was taken to three sets for a fourth time in the tournament, said.
"I feel like in the first set we were playing well, but then I rushed it a little bit. I'm just glad that I managed to win again in three sets."
