Coco Gauff and Caroline Garcia qualified for the WTA Finals after Aryna Sabalenka lost to Liudmila Samsonova in the Guadalajara Open second round on Wednesday.
Russian Samsonova, who has won 20 of her last 22 matches, smacked 21 winners and broke the powerful Belarusian four times en route to a 6-4 2-6 6-2 upset win over the world number four.
The WTA Finals, featuring the world's top eight players, starts in Fort Worth on Oct. 31.
Iga Swiatek, Ons Jabeur and Jessica Pegula had already locked up the first three spots before Guadalajara so with the addition of American Gauff and France's Garcia, three openings remain.
Fifth seed Gauff was not at her best but was still good enough to see off Italy's Elisabetta Cocciaretto 7-6(1) 6-3.
The American teenager double-faulted five times and converted just four of 13 break opportunities but was the better player on the biggest points, rolling through the first set tiebreak and carrying that momentum into the second set.
"I'm just super excited," said Gauff, 18, who will be the youngest player to compete at the WTA Finals since 2005.
"I wasn't really thinking about it before the match, but I'm excited to join Jess (Pegula)," she said. The pair have also both qualified for the WTA Finals doubles tournament.
Wimbledon champion Elena Rybakina's hopes of making the Finals ended after Pegula saved three match points to emerge with a 2-6 6-3 7-6(8) victory.
Rybakina, who fired down 14 aces against the American, would have qualified for the Finals if the WTA had issued ranking points at this year's grasscourt major.
The ATP and WTA stripped Wimbledon of ranking points after its organizers decided to ban Russian and Belarusian players in the wake of the invasion of Ukraine, which Moscow calls a "special military operation."
Twice major winner Victoria Azarenka also progressed to the third round after her top-seeded opponent Paula Badosa retired.
The 24-year-old Badosa, who appeared to be struggling to breathe, decided that she could no longer continue after losing the first set 6-2. The Spaniard's retirement means she can no longer qualify for the WTA Finals.
"You never want anybody to retire, it's really sad that Paula was unable to finish the match," Azarenka said.
Azarenka will next face Madison Keys, who battled back from a set down to beat Australian Ajla Tomljanovic 3-6 6-4 6-3.
"Madison is a great player, she's been playing really well this season, she can beat anybody," Azarenka said.
"We actually played in practice a couple of weeks ago and she killed me."
Danielle Collins and Jelena Ostapenko, who both have an outside shot of making the WTA Finals, won their second-round matches on Wednesday, while another hopeful Belinda Bencic was beaten 6-4 6-4 by Sloane Stephens.
