Aryna Sabalenka kept her hopes of ending the year as the World No 1 alive when she finished off Elena Rybakina to reach the semi-finals of the GNP Seguros WTA Finals Cancun on Friday in the last round-robin match of the Bacalar Group, which was suspended overnight due to rain.
We'll see what the conditions are going to be. Against Aryna it's never easy. I'm just gonna do my best and focus on myself and what I want to do on court. We usually have these matches that are really exciting and really tough, both mentally and physically. Iga Świątek
The resort of Cancun in Mexico suffered some 36 hours of wind and intermittent rain, and the night match was poised, with Sabalenka having won the first set and Rybakina leading 5-3 in the second, before it was halted at 11.30pm.
They returned to the Estadio Paradisus court some 16 hours later, when Rybakina levelled the match inside 2 minutes at a set apiece, before Sabalenka eventually won through, 6-2 3-6 6-3, to clinch her semi-final berth.
“Was really tough battle in really tough conditions,” Sabalenka said afterwards. “I’m super happy that I was able to get this win no matter what.”
Sabalenka is now a step closer to finishing as the year-end No 1, and, if she wins her Saturday semi-final, she will lock it down, but should the Belarusian lose, and Iga Swiatek goes on to win the title here, the 22-year Pole will regain the top ranking she held for 75 weeks before her losing in the Last 8 at the US Open.
The first game of the decider was a monumental test of wills, with Sabalenka surviving 9 deuces and 2 break points in a 17-minute hold of serve.
With Rybakina serving at 1-2, Sabalenka unleashed a pair of forehand winners to take the early lead, but the Kazakh got herself back on serve, aided by a top-seed double-fault.
Sabalenka then secured the 3rd straight break of the set to love, taking a 4-2 lead, and served for the match at 5-3, fighting off 5 break points before converting her 4th match point with a thundering ace to land the 2-hour 27-minute contest.
A runner-up at the WTA Finals last year in Fort Worth, Sabalenka is looking to become the first player to reach consecutive finals since Serena Williams in 2013 and 2014.
“I think I don’t have to compare this WTA Finals to last year,” Sabalenka said. “It’s completely two different tournaments, two different conditions.
“I would just say that this year I have to focus on myself and don’t let all these [No.1] thoughts, these challenges go to my mind and, kind of, like, destroy myself.”
Sabalenka now must face Swiatek in the semi-final, after the Pole won the Chetumal Group on Friday night, emphatically seeing off Ons Jabeur, 6-1 6-2, in just 67 minutes to finish the round-robin stage, 3-0.
Swiatek maintained her dominance over Jabeur, winning 77% of her first-serve points to the latter’s 60%, while the Pole also won 81% or 17 of 21 points played on return on the Tunisian’s second serve, and 59% of her second-serve points.
Despite her struggles on her second serve, Jabeur did bring up 5 break points on Swiatek’s serve, but could only convert one of these.
Swiatek converted 5 break points on the Tunisian’s serve, while the win meant that the World No 2, like Jessica Pegula in the Bacalar Group, has moved on ahead in the tournament on an undefeated note.
The semi-final is a rematch of last year’s at Fort Worth, which Sabalenka won, 6-2 2-6 6-1, and the two have split their 2 meetings this season, both on clay, with Swiatek winning the Stuttgart final, while the Belarusian took Madrid.
“We’ll see what the conditions are going to be,” Swiatek said. “Against Aryna it’s never easy. I’m just gonna do my best and focus on myself and what I want to do on court. We usually have these matches that are really exciting and really tough, both mentally and physically.”
Earlier, Coco Gauff, the World No 3, secured her spot in the semi-finals by getting past Marketa Vondrousova, 5-7 6-7(4) 6-3, after 2 hours and 28 minutes in the final round of group play on a wind-strewn Friday evening.
The win eliminated the Czech Wimbledon champion’s hopes of advancing to the Last 4, while Gauff is the first teenager to make the semis of the year-end championships since Caroline Wozniacki in 2009.
The Czech proved to be a tough opponent as she raced out to a 3-0 lead in the first set, but Gauff, the US Open champion, fought back to level at 5-5.
While Vondrousova, though, held her nerve to take the opener, Gauff dominated at the start of the second set, winning the first 5 games, but she failed to serve out the set at 5-3, and the Czech found herself 4 points from victory at 6-5.
Gauff played aggressively to win that game and forced the match into a breaker and a third set, which she eventually won to register her 51st win of the season.
“I felt like I needed to hit through the court,” The 19-year old American said afterwards. “She’s a very tricky player. When you play her pace and let her move you side to side it’s tough. And I played her, it’s my third time, so I know that.
“But obviously, in the wind, it’s tough to be as aggressive as you want, because the bounces are weird and the ball was moving. So I’m glad I made that adjustment and didn’t get too frustrated when I missed. You pay the price for hopefully a good reward.”
Gauff, who has now won 24 of her last 27 matches, all under coach Brad Gilbert, will now face her friend and doubles partner Pegula.
Pegula, ranked 5, is 3-0 this week, and leads Gauff 2-1 and 2-0 on hard courts.
It will be the first all-American semi-final at the WTA Finals since the reintroduction of the round-robin format in 2003.
Both are making their semi-final debuts at the season-ending championships after going winless in both singles and doubles last year in Fort Worth.
This will be the third meeting of the year between the two top-ranked Americans. Gauff won on the grass in Eastbourne and Pegula got her revenge during her title run in Montreal this summer.
Saturday’s schedule, which includes one remaining round-robin doubles match that will determine the 4th semi-finalists:
7.30 pm (all GMT)
 Gabriela Dabrowski & Erin Routliffe vs.  Nicole Melichar-Martinez & Ellen Perez
Not Before 10.00 pm
 Jessica Pegula vs.  Coco Gauff
Not Before 11.00 pm
 Aryna Sabalenka vs.  Iga Swiatek
 Coco Gauff & Jessica Pegula vs.  Laura Siegemund & Vera Zvonareva (to finish, 6-3, 1-1
After suitable rest
TBC vs.  Storm Hunter & EliseMertens