Jessica Pegula prevailed against US Open winner and doubles partner Coco Gauff and secured her place in the final of the season-ending WTA Finals in Mexico.
Pegula beat her American compatriot 6-2, 6-1 in an hour in wet and windy conditions in Cancun.
The other semi-final between Igi Swiatek and Aryna Sabalenka was postponed to Sunday due to rain.
When the game was canceled, Swiatek was leading 2-1 and the final was postponed to Monday.
Poland’s Swiatek could usurp Sabalenka at the top of the women’s rankings if she beats the Belarusian and wins the event.
Sabalenka will retain her world ranking if she reaches the final.
World number five Pegula, 29, took advantage of six of her 10 break point chances and only lost her serve once against 19-year-old Gauff, who sits third in the women’s rankings.
“It’s probably my biggest final ever,” Pegula said after her win. “This will give me a lot of confidence for next year.”
Rain broke play twice in the second set, but Pegula dominated throughout against Gauff, who was hampered by 25 unforced errors.
“I did a really good job of doing what I wanted,” Pegula said. “I think it worked even better in the windy conditions.
“I just tried not to get frustrated with my serves or my returns or any weird points we had and just keep moving.”