Jessica Pegula continued her superb form at the WTA Finals on Saturday by thrashing fellow American and doubles partner Coco Gauff 6-2, 6-1 to reach the final in Cancun, Mexico.
The other semi-final - a heavyweight contest between world No 1 Aryna Sabalenka and second-ranked Iga Swiatek - was suspended because of rain.
Sabalenka and Swiatek had played just three games when rain blew in, halting a match between players fighting for the year-end No 1 ranking.
If Australian Open champion Sabalenka wins, she will finish the year top of the rankings, but if French Open champion Swiatek advances and goes on to claim the title she will regain the No 1 spot.
Whoever wins will have to get past a red-hot Pegula, who has not lost a set on the outdoor hardcourts in Cancun, following impressive group-stage wins over Elena Rybakina, Maria Sakkari, and Sabalenka.
Pegula then shook off a late rain delay and windy conditions to dispatch US Open champion Gauff in an hour.
"I executed what I wanted to do really well," Pegula said. "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."
There was no respite for Sabalenka and Swiatek from the weather that has wreaked havoc all week at the tournament.
They were on serve with Swiatek up 2-1 in the opening set when showers started, the players remaining on the court wrapped in towels amid hopes they would get underway again soon.
However, heavier rain saw them sent to the locker room, officials calling a halt to the night shortly after 10pm local time.
Sabalenka beat Swiatek in last year's WTA Finals semi-finals and has split two clay meetings with the Pole this year, winning at Madrid while falling to Swiatek in the final at Stuttgart.
Pegula is 2-4 in her career against Sabalenka, but beat her earlier this week in round-robin play, and 3-5 against Swiatek.
Gauff, who took her first Grand Slam title in September on the New York hardcourts, and Pegula met in the first all-US semi-final at the WTA Finals since a round-robin format for the event was restarted in 2003.
Pegula, who captured her seventh consecutive triumph over a top-10 rival, won 10 of the last 11 games and broke six times in a dominating performance, stretching her overall win streak to nine matches.
"The wind is so tough, I feel like I hit hard and flat. If I do that up the middle it's hard for people to create," Pegula said. "If you're going to go out and hit winners in this it's ,like, too good.
"But I don't think anyone can really do that that efficiently."
Windy and rainy conditions for many matches and issues with the playing surface brought complaints from players all week, prompting WTA Tour chief executive officer Steve Simon to respond to players.
"It is clear you are not happy with the decision to be here in Cancun. I understand that and you have been heard," Simon said. "It is not a perfect event. We understand the conditions are a challenge and the WTA accepts responsibility for that."