Jessica Pegula progressed to the last four at the WTA Finals with a perfect record but Aryna Sabalenka and Elena Rybakina must resume their match on Friday after play was suspended.
American Pegula had already qualified but the fifth seed made it three successive victories in Cancun by beating Greek Maria Sakkari 6-3 6-2.
World number one Sabalenka led 6-2 3-5 against Rybakina, who will serve to level the match when play resumes.
The winner will reach the semi-finals.
Also on Friday, Iga Swiatek faces Ons Jabeur, while Coco Gauff plays Marketa Vondrousova to determine who will progress from the Chetumal Group.
Swiatek is within touching distance of the last four after winning both of her matches – but all four players could still yet qualify.
Pegula raced into a 5-1 lead against Sakkari on her way to taking the opening set and, on a wet and windy night, the 29-year-old was able to see out victory after winning the first four games in the second.
It is the first time Pegula has qualified for the semi-finals at the season-ending tournament, and she is the first American to reach the final four since Sloane Stephens in 2018.
“Getting through 3-0 is really kind of a cool accomplishment, especially coming off of last year, where it was the opposite,” Pegula said.
Meanwhile, Belarusian top seed Sabalenka had taken charge of her crucial match against Rybakina, clinching the opening set with a double break.
But Kazakhstan’s Rybakina will serve to force a third set when they resume on Friday after coming back from a break down to lead 5-3 in the second.
The players left the court at 23:40 local time – more than three hours after the match began – at the end of an evening interrupted by bad weather.
Sabalenka is set to retain her world number ranking if she reaches the final.