With Maria Sakkari's win against Aryna Sabalenka in a terrific three-set match Monday night in Guadalajara, the semifinals are set for this year's WTA Finals. 

First up Tuesday: The first all-Spaniard final four showdown at the year-end championships. Paula Badosa will square off against Garbiñe Muguruza. They've never played, and the truth is when you balance form and experience, this one is hard to call. 

At night, Anett Kontaveit, who has been the hottest player on tour, takes on Sakkari, who needed 2 hours and 47 minutes in her last match to escape the round-robin stage. 

Will she have enough juice left? Will Kontaveit feel any nerves? 

Two terrific matchups await, with the winners one step closer to the coveted year-end title. How will it play out? We break it down. 

Paula Badosa vs. Garbiñe Muguruza

Key for Badosa: Stay calm and composed

With Anett Kontaveit’s rush to qualify for the WTA Finals eclipsing most other season-ending news, it was nearly forgotten Paula Badosa also came into Guadalajara on an undefeated run.

Badosa’s eight-match winning streak finally ended with a loss to Iga Swiatek in her last round-robin match, which happened to fall on her 24th birthday. However, this week, Badosa has credited her recent form to improving mentality and maturity.

“[A] few years ago, mentally I was a typical player,” Badosa said Saturday. “‘Oh, she can play good, but mentally she's so far away. She needs to improve a lot on that.’ I listened to that a lot.

“Now I think I'm maybe one of the best ones, or that's what I try. ... Of course, I improved on my tennis and physically. That's tough. But I think the toughest part to improve is on the mental game.”

That improvement has been noticeable all year. At the start of the season, Badosa had no WTA singles titles and no Top 10 wins; now she has two titles and six wins over Top 10 opposition.

Badosa kept it up this week.

Mentality: Grabbing five break points against Aryna Sabalenka, she converted every one in her rout of the No.1 seed.

Maturity: She had two fewer winners but 27 fewer unforced errors than No.4 seed Maria Sakkari while gritting out two close sets.

Continuing to be mentally strong and composed will be the key to defeating fellow Spaniard Garbiñe Muguruza in the semifinals. Muguruza is more experienced at the top level, but Badosa has another year of learning and growing under her belt.

“Today I'm getting older, so that's maybe helping,” Badosa said with a smile. – Jason Juzwiak

Key or Muguruza: Maintain that free-spirited energy

Ahead of the WTA Finals, Garbiñe Muguruza posted an Instagram video of herself dancing insouciantly around the town of Tequila, an hour's drive northwest of Guadalajara. She was soundtracked by Pete Rodriguez's 1967 single "I like it like that," a classic boogaloo track that's also a staple of Latin American pop culture.

The Spaniard has brought the same free-spirited energy into her fourth appearance at the year-end finale. Muguruza has played with heart and passion all week, feeding off the fervent support she's received from the Mexican crowd to elevate her game - the kind of symbiotic crowd-player relationship that can make sport so special.

This was the case even when she lost a heartbreaker in her opening match to Karolina Pliskova. It was even more evident when Muguruza bounced back to defeat Barbora Krejcikova, to whom she had lost twice this year, and then to end Anett Kontaveit's 12-match winning streak to seal her semifinal spot.

"The Latin America environment, air, people, crowd, everything," Muguruza said after beating Kontaveit. "I love it here. I feel so home and welcome. People are very good with me. I guess that shows in my tennis. It is very special for me. This is once-in-a-lifetime opportunity for me to play the Finals here. I'm very, extremely motivated, I would say."

Muguruza's semifinal against Paula Badosa is a first-time encounter against a compatriot with whom she bonded as Olympic teammates this year. All of that makes for several fascinating sub-threads to their match, and could mean that the crowd's favour will be more evenly split.

But Muguruza is feeling inspired by something larger than her: the rich history of Latin American culture. She is perfectly in sync with everything around her. That's the kind of emotional state that can count for more than matchups and technique.

There's been much talk of how the conditions of Guadalajara will affect results. Muguruza is demonstrating that the psychogeographic aspect to the location could be even more important. – Alex Macpherson

Anett Kontaveit vs. Maria Sakkari

Key for Kontaveit: Stay aggressive and strike cleanly

Anett Kontaveit came into the Akron WTA Finals Guadalajara on an absolute tear. In the two months after the US Open, she won three titles and 19 of 20 matches. Of course, her unparalleled momentum came at a cost.

“I think we’ll see when we start playing matches if it’s worked in my favor or not,” Kontaveit told reporters. “I mean, I’m really, really trying to focus on what I have to do. I’ve played so many matches – I don’t think it’s a bad thing. I like it. I mean, I think it’s a good position to be in.”

Read more: Anett Kontaveit and her remarkable turnaround this season

She was right, winning her first two round-robin matches over Barbora Krejcikova and Karolina Pliskova in straight sets and bounding into the semifinals.

Anett Kontaveit

Jimmy48/WTA

Her semifinal opponent, Maria Sakkari, played far fewer matches since the US Open – 11. But there are extenuating circumstances. While Kontaveit wrapped up round-robin play Sunday night, Sakkari didn’t complete her third match until more than 25 hours later. It was a thrilling 7-6 (1), 6-7 (6), 6-3 win over Aryna Sabalenka but required 2 hours, 47 minutes.

Sakkari holds a tenuous 6-5 head-to-head edge. Kontaveit, though, won their last meeting, in the Ostrava final.

With all the matches she's played, Kontaveit needs to make sure she maintains the energy that has come with her success the past couple of months. She will also need to play aggressive and, as Chris Evert said, maintain clean ball-striking.   

In Kontaveit’s final round-robin match, a straight-sets loss to Garbiñe Muguruza, the 25-year-old Estonian looked understandably weary.

Does she enough in her tank to win two more?

“I wish had a way to measure it,” he coach, Dmitry Tursunov said Monday night. “Definitely, I would agree that she looked a little more tired. But when your back is against the wall, you play different tennis. We’ll see tomorrow.”

Kontaveit’s back has been up against it for two months, and she’s performed remarkably well. As Tursunov pointed out, the loss to Muguruza had no qualifying implications for Kontaveit since she was already in. As for that head-to-head? Kontaveit was 0-3 previously against Pliskova before slamming her in straight sets in round-robin play. – Greg Garber

Key for Sakkari: Land your serves and run 

Looking at the 11 previous matches between Maria Sakkari and Anett Kontaveit reveals very little other than the dead-heat that exists between these two good friends. Only one of their previous 11 meetings even went past two sets. It came in February at the Grampians Trophy in Melbourne, and that wasn't even a three-setter. Kontaveit prevailed in a super-tiebreak on a dead netcord on match point, winning it 11-9. 

Sakkari will have the advantage of coming into Tuesday's match having already tallied a night-session win, a 2 hour and 47 minute 7-6(1), 6-7(6), 6-3 win over top seed Sabalenka on Monday night. Sakkari did well to adjust and find the measure of the court, whereas Kontaveit looked puzzled with her inability to dial in her shots in her 6-4, 6-4 loss to Muguruza the previous night. 

To get the win over Kontaveit, Sakkari will need her lethal first serve. In her two wins this week over Iga Swiatek and Sabalenka, Sakkari won in excess of 70% of her first serve points. When she lost to Badosa, she barely won over 50%. Sakkari will need to keep Kontaveit at bay in the rallies and that begins with rolling through her service games and not letting Kontaveit unleash on her second-serve return.

Once the rallies get started, Sakkari will need to dig into her reserves and run. There will be questions surrounding how well she can recover from her three-hour effort against Sabalenka, but Sakkari will know that Kontaveit's astounding finish to the season has also had an effect on the Estonian. Building rally pressure and earning errors will tip things quickly in Sakkari's favor, especially if she's serving well. 

"I'm getting used to them," Sakkari said. "This is how it's going to be from now on because if I want to play against these players and win, I'm not going to win 1-1. I know I'm going to have to take it to a third set most times and do something special to beat them."

Regardless of the outcome, keep an eye on the post-match handshake. These two began their Slam seasons in hard quarantine in Melbourne and kept each other entertained on daily FaceTime calls. Eight months later, they'll be facing off for the biggest final of their respective careers. Regardless of the outcome, expect a big hug of relief and appreciation. – Courtney Nguyen