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.
Pegula won 6-2 6-1 in one hour against her fellow American in wet and windy conditions in Cancun.
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.