Former world number one Karolina Pliskova earned a place in the year-ending WTA Finals despite a second-round defeat by Russian veteran Vera Zvonareva at Moscow's Kremlin Cup on Wednesday.
Pliskova's 6-1, 6-2 loss to Zvonareva, the 2010 Wimbledon and US Open runner-up, also secured Elina Svitolina's spot in the eight-woman field in Singapore next week.
Czech star Pliskova's participation was confirmed after fourth seed Kiki Bertens lost 6-3, 4-6, 6-3 to Aliaksandra Sasnovich of Belarus.
"I'm delighted to be appearing at the WTA Finals for the third time," Pliskova said.
"I have such great memories from playing in Singapore in 2016 and 2017, and I can't wait to close out the year competing against the other elite players of 2018."
Pliskova and Svitolina joined top-ranked Simona Halep of Romania, Germany's Angelique Kerber, Japan's Naomi Osaka, Czech Petra Kvitova, Caroline Wozniacki of Denmark and Sloane Stephens of the USA.
They will battle for a $7 million (6.1 million euros) prize purse in Singapore.
Bertens, who sits ninth in the race to Singapore rankings, could still feature at the season-ending event as Halep, who pulled out of the Kremlin Cup on Tuesday, is battling a back injury.
She was diagnosed with a herniated disk following her withdrawal from Beijing two weeks ago.
The 34-year-old Zvonareva, who was unranked last year after taking a two-year hiatus to have her first child, claimed her first victory over a top-10 player since beating Caroline Wozniacki at the WTA Tour Finals in 2011.
"I knew that matches against top-10 players never come easy and tried hard to capitalise on every single opportunity that I had today and it paid off," the current world number 161 said after downing Pliskova.
Third-seeded American Sloane Stephens, the French Open finalist and former US Open champion who qualified for the WTA Finals earlier this week, also slumped to a 6-3, 6-2 loss to Tunisian qualifier Ons Jabeur.
In the quarter-finals, Jabeur will face eighth seed Anett Kontaveit, who saw off Frenchwoman Kristina Mladenovic 6-2, 7-6 (7/3).
In the men's draw, left-handed Frenchman Adrian Mannarino cruised into the last eight past top-seeded Italian Marco Cecchinato, beating this year's French Open semi-finalist 6-2, 6-3 in just 50 minutes.
The 30-year-old Mannarino will next meet Belarusian qualifier Egor Gerasimov, who beat Benoit Paire 6-4, 6-4.