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 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.
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.
Sevastova beats Zvonareva
Anastasija Sevastova reached the Kremlin Cup semifinals by beating Russian veteran Vera Zvonareva 4-6, 7-5, 6-3 on Thursday.
The Latvian said she was lucky not to lose the match after Zvonareva - once ranked second in the world but in her first tour quarterfinal since 2012 - broke her serve to level the second set at 5-5.
"I started playing better and better and toward the end I was playing one of my best matches here in Moscow," said Sevastova, the highest-ranked player left in the draw at No. 11.
Australian Open semifinalist, Sevastova will next face Tunisian qualifier Ons Jabeur, who upset eighth-seeded Anett Kontaveit 7-5, 6-1.
Jabeur has won six straight matches, including qualifiers. She has reached her first WTA semifinals.
Jabeur was the 2011 French Open junior champion but has struggled to live up to her early potential.
Nick Kyrgios withdrew from the men's tournament with an elbow injury, giving Mirza Basic a walkover to the quarterfinals, where he'll play Karen Khachanov.
Kyrgios said it was a recurrence of an injury which caused him to miss several tournaments around and including the French Open.
"I'm going to choose to wrap up my season, head home and start the recovery," the Australian said. "See you all in 2019."
Second-seeded Daniil Medvedev reached the quarterfinals with a 6-2, 6-1 win over Dusan Lajovic in just 51 minutes. He will next face last year's defeated finalist Ricardas Berankis.