Garbine Muguruza dominated Paula Badosa in a 6-3, 6-3 victory and will play for the WTA Finals title for the first time in her career.
The sixth-seeded Muguruza, who lost in the semifinals in 2015, is the first Spaniard to reach the singles final since 1993, when Arantxa Sanchez-Vicario was runner-up to Steffi Graff.
“I’m very happy with my performance, it was the best match I played here in Guadalajara,” Muguruza said.
"It's the first time we encountered each other. I'm very proud of Paula, she started the year back in the rankings and made all the way to the top 10."
The 28-year-old Spaniard now has a 13-2 record on Mexican soil, where she won back-to-back titles in Monterrey in 2018 and '19.
Mugurza will take on eighth-seeded Anett Kontaveit, who beat fourth-seeded Maria Sakkari 6-1, 3-6, 6-3 in the other semifinal.
Muguruza started the season-ending tournament with a loss to third-seeded Karolina Pliskova but bounced back with victories over second-seeded Barbora Krejcikova, Kontaveit and Badosa.
I don't feel like a favourite, maybe on paper or for the TV, but I don't feel like it. I started here losing and I thought that I could be eliminated, so for me to say that I feel like a favourite for the final is complicated."
—Garbine Muguruza
"I don't feel like a favourite, maybe on paper or for the TV, but I don't feel like it," she said.
"I started here losing and I thought that I could be eliminated, so for me to say that I feel like a favourite for the final is complicated."
The Spaniard reached 41 victories this season, her most in a single campaign since 2017 when she finished with 45.
Muguruza broke the seventh-seeded Badosa in the third game and then held to win the first set in 35 minutes.
In the second set, Muguruza had a break in the second game, saved a couple of break points in the third and never looked back.
Badosa, who turned 24 on Monday, was the youngest player in the semifinals. The Spaniard finished a breakthrough season that saw her crack the top 10 for the first time in her career.
