Jessica Pegula romped past US Open champion and doubles partner Coco Gauff to secure her place in the final of the season-ending WTA Finals in Mexico.

The other semi-final between Igi Swiatek and Aryna Sabalenka was postponed until Sunday because of rain.

Swiatek led 2-1 when the match was called off, with the final rescheduled to take place on Monday.

Poland's Swiatek could usurp Sabalenka at the top of the women's rankings if she beats the Belarusian and goes on to win the event.

Sabalenka is set to retain her world number one ranking if she reaches the final.

World number five Pegula, 29, took six of her 10 break point chances and lost serve just once against 19-year-old Gauff, who is third in the women's rankings.

"It's probably my biggest final ever," said Pegula following her victory. "It's going to give me a lot of confidence going into next year."

Rain interrupted play twice in the second set but Pegula dominated throughout against Gauff, who was hampered by 25 unforced errors.

"I executed what I wanted to do really well," said Pegula. "In the windy conditions, I think it worked even better.

"I was just trying not to get frustrated with my serve or returns or any funky kind of points that we had and just to keep my feet moving."