Swiatek will face either American Danielle Collins or Croatia's Donna Vekic for the title after their semi-final clash was suspended following three rain delays and scheduled to resume on Sunday.

Collins was two games away from a hard-fought victory when showers forced players off for a final time with a 4-6 6-4 4-2 lead over Vekic.

Reigning US Open champion Swiatek sprinted out to a 4-2 lead in the first set before Pegula roared back, reeling off four straight games and capturing the first set on Swiatek's 11th forehand error of the frame.

Rain postponed the start of the second set by more than an hour and the break appeared to boost Poland's Swiatek, who was sharper in the second set, breaking Pegula with a forehand winner on the line to level the contest at a set apiece.

In the third set, Swiatek raced to the net to catch up with a drop shot and sent a backhand down the line for a 4-2 lead she would not relinquish, improving to 4-1 lifetime against Pegula when the American's shot landed in the bottom of the net on match point.

"I was leading in the first set so I knew I had the game to win this match but I lost my focus in the middle of the set," Swiatek said in an on-court interview.

"I didn't want to make that mistake again so I came back and I wanted to be really composed and not lose those important moments," she said, adding that she spent the rain delay doing crossword puzzles.

Swiatek will look to claim her eighth title of the season on Sunday.

With her run to the semi-finals of the WTA 500 event, Pegula will break into the world top five when rankings are released after the final.