CANCUN, Mexico — This is one of the most dynamic matchups the Hologic WTA Tour has to offer: World No.1 Aryna Sabalenka versus No.4 Elena Rybakina.
On Thursday evening, it’s winner-take-all between Sabalenka and Rybakina for a spot in the semifinals at the GNP Seguros WTA Finals Cancun. Both are 1-1 through two matches at this round-robin event.
“Now it’s going to be either, you win and you stay or you lose and you go back home,” Rybakina told reporters Tuesday after surviving the longest match of the tournament. “It’s not going to be easy, and I’m going to try to do my best.”
Back in January, they met in the Australian Open final, and Sabalenka crashed through with a three-set victory for her first major. That gave the 25-year-old a 4-0 career record over Rybakina.
Since then, the 24-year-old from Kazakhstan has reversed the curse and won two straight matches. The Indian Wells final in March was a 7-6 (11), 6-4 thriller, and less than a month ago, Rybakina prevailed again, 7-5, 6-2, in the Beijing quarterfinals.
There’s a dramatic subtext, too: With Swiatek still in pursuit, Sabalenka’s No.1 ranking is still very much in play.
Settle in and enjoy the festivities as the Bacalar Group concludes its business with the last of three round-robin matches.
 Aryna Sabalenka vs.  Elena Rybakina
The case for Sabalenka: This one features two of the hardest hitters on the Hologic WTA Tour, players ranked second and third in aces.
After a scorching 6-0, 6-1 opening win against No.8 seed Maria Sakkari, Sabalenka regressed dramatically in a 6-4, 6-3 loss to group winner Jessica Pegula. Sabalenka won barely half of her first-serve points and was broken five times. She had only a single ace — against five double faults. Sabalenka will have to do better against Rybakina, who will be buoyed by her late victory over Sakkari.
Sabalenka, who lost in last year’s WTA Finals championship to Caroline Garcia, remains intent on reaching the next level, and this is the critical step. She can equal her career-best of eight Top 10 wins in a single season with a victory over Rybakina.
By the time Sabalenka arrived for a press conference following the loss to Pegula, less than an hour had passed, yet she seemed to have already wiped the result from her memory.
“I’ll do some recovery tonight,” she said, “just try to analyze what happened today. Try to learn this lesson as soon as I can.”
The case for Rybakina: As miserable as she performed in her opening match against Pegula, Tuesday’s second round-robin match was a vast improvement.
Rybakina was a 6-0, 6-7 (4), 7-6 (2) over Sakkari, marking the first three-set singles match at this year-end event and the longest at 2 hours, 24 minutes.
She is keenly aware of the nuances in Sabalenka’s game; this is their seventh meeting.
“I’m sure it’s going to be a tough, tough match again,” Rybakina said. “I think we’re both going to fight and we see how it’s going to go.”
 Jessica Pegula vs.  Maria Sakkari
The case for Pegula: This match doesn’t have any real implications outside of momentum, ranking points, and pride. Pegula was seeded behind Sabalenka and Rybakina in the Bacalar Group, yet she emerged as the undefeated winner to advance to the semifinals, while Sakkari, who lost her first two round-robin matches, has been eliminated.
Coming in, much was made of Pegula’s inaugural WTA Finals, when she went 0-6 last year in Fort Worth in singles and doubles. But at 29 — she’s the oldest singles player in the field, born six months before Ons Jabeur — she is wiser. Maybe it’s the fact she’s played 120 singles and doubles matches this year.
“I don’t know if it’s experience or like 5,000 matches later, whatever it is, but I think I felt a lot more comfortable,” Pegula said. “I feel more comfortable playing [top players] this year, I feel more confident,” she told reporters afterward. “I don’t think it feels like such a big deal. Last year when I made Top 5, I think I psyched myself out a little bit, not a lot. You have a couple thoughts in your head — do I really belong at that level?
“I was always good at beating those below me, but now you have to beat the one or two before you. I think you put a bigger emphasis on that. And I think this year I’ve not really cared.”
The case for Sakkari: Say this for the 28-year-old from Greece — she is a gamer.
Against Rybakina on Tuesday, she dropped her second consecutive opening set 6-0 and then came back to push Rybakina to the brink.
“My biggest win today was that I overcame that fear on the court. I was points away from perhaps winning that match,” Sakkari said afterward. “I thought that I was going to go down as not playing good and finishing the season not in a good way, but today’s performance was promising.
“One side of me was telling me that I cannot do it, but then I think another side of me was also … like I had two creatures fighting each other in my mind and within myself. I’m happy that I managed to overcome this.”
These two have a history, and it favors Sakkari; she holds a 5-4 career advantage.