Day 4 of the Akron WTA Finals Guadalajara sees Group Chichén Itzá reach its midpoint. One match will be a showdown for a semifinal place, while the other a battle to avoid elimination.
Maria Sakkari and Paula Badosa both kicked off their WTA Finals campaigns with straight-set victories, and the winner will be that much closer to solidifying a spot in the semis. Meanwhile, the loser of Aryna Sabalenka's tie with Iga Swiatek will be out of contention for the final four.
If Sakkari beats Badosa and Swiatek beats Sabalenka, the Greek will top the group; Swiatek and Badosa would play Monday for second place. If Badosa beats Sakkari and Sabalenka beats Swiatek, the Spaniard will top the group; Sakkari and Sabalenka would play for second place.
If Sakkari wins and Sabalenka beats Swiatek in three sets, she will still advance to the semifinals, but her group position would still be undetermined. If Badosa wins and Swiatek beats Sabalenka in three sets, the same would apply to her.
Any other combination of results will mean that semifinal advancement will come down to Monday's final round-robin matches.
 Maria Sakkari (GRE) vs.  Paula Badosa (ESP)
Every player in the WTA Finals field has taken the court, and it's clear one of the most important shots of the week will be the serve. The difficulty in controlling groundstrokes is exacerbated on the return, a shot that requires pinpoint timing to execute aggressively. Players whose strongest serves are clicking can rattle through quick holds to keep their opponent at bay.
That's particularly been the case for Sakkari. The improvement in her serve has arguably been the most crucial element of her transformation into a Top 10 player this year, and against Swiatek it was at its best. The 26-year-old Sakkari won an astonishing 26 of 27 first-serve points and faced break points in only one game.
Badosa wasn't far behind. After falling behind 4-2 against Sabalenka, she would lose only another eight points on serve as she reeled off the last 10 games of the match.
Both Sakkari and Badosa are continuing to prove the value of hard work this season. Their paths to their WTA Finals debuts were gradual, but marked by a commitment to the kind of continuous improvements and additions that only come through sheer graft. As a result, the quality they now bring to the court is unrecognisable from the players they were even 12 months ago.
Finding their best tennis has not come easily. Perhaps that is why both have managed to adjust to the Guadalajara conditions so well. Before the tournament, Sakkari stated the winner would be "whoever accepts the most mistakes, whoever accepts playing ugly tennis." That attitude comes naturally to a player who has fought and scrapped her way up the rankings as a baseline grinder, even if she now has superior weaponry in her repertoire.
Badosa echoed Sakkari after defeating Sabalenka for the second time this year. "I know and I have to accept that I'm going to miss more than normal," she said.
Both players arrived in Guadalajara with positive preparation. For Badosa, it's been an unexpected whirlwind after her Indian Wells victory catapulted her into contention - but the confidence from that title run is still with her, and she's now on a seven-match winning streak. For Sakkari, smart scheduling has been another significant component of her season.
"We've been very clever with [coach] Tom [Hill] and the rest of my team on getting the right amount of rest before every tournament, having many training blocks throughout the season," she said. "Even before coming here, I had six days off from tennis, which was something that I really needed."
Sakkari and Badosa are both juggernauts at the moment, able to play close to their highest level when the stakes demand it. It's fitting that this first-time encounter will have a semifinal spot on the line.
 Aryna Sabalenka (BLR) vs.  Iga Swiatek (POL)
If Sakkari and Badosa were supreme, their beaten Day 2 opponents, Swiatek and Sabalenka, will need to go back to the drawing board. Both failed to adjust to either their opponent or the conditions. Swiatek racked up 29 unforced errors and lost her sixth consecutive set to Sakkari, while Sabalenka committed 31 unforced errors in just 16 games against Badosa.
Both opened up about their inability to deal with the pressure of the match afterward.
"After I lost the serve, I was really disappointed [in] myself and emotionally I was really crazy," Sabalenka said. "I couldn't just stop myself and put myself back in the match."
Swiatek, who was visibly emotional before match point, told a similar tale - with the added wrinkle of her previous losses to Sakkari still in her head.
"I was really stressed and it was hard for me to understand, because I was playing pretty well in my practices," Swiatek said. "When I got broken in the first set, the vibe on court totally changed. I remembered how it was in Ostrava and Roland Garros when Sakkari broke me, and I had flashbacks from that matches. I know it isn't really constructive, but I guess the stress caused that I wasn't able to come back [with] better thoughts, to just be more focused."
Swiatek also said she had still not figured out how to play Sakkari.
"She can attack as well as stay back and just wait for her moment. I think she has made huge progress in that. It's tricky for me to know when to risk or just when to hold the ball."
The first meeting between Swiatek and Sabalenka would be a popcorn one in most circumstances. Both possess impressive power, but Swiatek's addition of heavy spin facing Sabalenka's brute force makes this one intriguing. These are two of the most overwhelming games on tour when they're at their best. Pitted against each other, whose strengths will win the day?
Both Swiatek and Sabalenka have championed the use of sports psychologists this year - Sabalenka's use of one saw her break through a Grand Slam barrier to two semifinals, while former sailor Daria Abramowicz has long been an integral part of Swiatek's team. They will need to draw on everything they've learned to bounce back.
AKRON WTA FINALS GUADALAJARA: DAY 4 ORDER OF PLAY
Starting at 2 p.m.
 Maria SAKKARI (GRE) vs  Paula BADOSA (ESP)
 Barbora KREJCIKOVA (CZE) / Katerina SINIAKOVA (CZE) vs  HSIEH Su-Wei (TPE) / Elise MERTENS (BEL)
Not before 7.30 p.m.
 Aryna SABALENKA (BLR) vs  Iga SWIATEK (POL)
 Alexa GUARACHI (CHI) / Desirae KRAWCZYK (USA) vs  Sharon FICHMAN (CAN) / Giuliana OLMOS (MEX)