World No 1 Aryna Sabalenka defeated Elena Rybakina 6-2, 3-6, 6-3 in a rain-disrupted group match on Friday to reach the semifinals of the WTA Finals.
The Belarusian, who finished second in her group behind unbeaten American Jessica Pegula, kept her bid for the year-end No 1 ranking on track with a victory that took two days to complete.
She had won the first set but was trailing in the second when play was halted on Thursday night after multiple rain delays.
Some 16 hours later, she and Rybakina returned to the outdoor hardcourts in Cancun, where fourth-ranked Rybakina of Kazakhstan calmly served out the second set to level the match in a matter of minutes.
Australian Open champion Sabalenka dug deep to halt Rybakina's momentum in the opening game of the third set, surviving nine deuces and two break points to hold serve.
She used a pair of ferocious forehands to break Rybakina in the fourth game.
If Sabalenka wins her semifinal match on Saturday, she's assured of finishing the year at No 1.
If she falls, she'll open the door to Poland's Iga Swiatek – who topped the rankings for 75 weeks but dropped from the summit after the US Open.
Swiatek, 2-0 in group play, faced Tunisian Ons Jabeur later in her last group match, but was already assured of a semifinal berth.
A win by Jabeur would see the Tunisian win the group and Swiatek advance as runner-up.
US Open champion Gauff, who closed out group play with a 5-7, 7-6 (7/4), 6-3 victory over Wimbledon winner Marketa Vondrousova in another rain-disrupted match, could still advance with a Swiatek win over Jabeur.
In windy conditions, Vondrousova won 12 of the first 13 points and broke twice for a 3-0 lead before Gauff rallied to 3-3.
They traded breaks over the next four games to 5-5 before rain halted the match. When they resumed, Gauff surrendered a break on a double fault before her Czech rival sealed the first set with an overhead smash.
After being broken to begin the second set, Gauff rolled to a 4-1 edge. But Vondrousova battled back to 5-5 and they exchanged breaks to reach the tiebreaker.
Gauff's aggressive approach paid off in the tiebreaker and she continued it in the third set, breaking Vondrousova at love for a 3-2 lead and rolling to victory.