Elena Rybakina completed a clean sweep of her WTA Finals group with a straight sets win over late Callina Alexandrova.
The sixth seed, who sealed his progress to the semi-finals on Monday, continued his momentum with a 6-40 win in Riyadh.
The Kazakh was due to play the keys in Madison, but the American – who could not improve – quit hours before the start of the match due to illness.
Alexandrova was the second alternate, but received the nod after fellow Russian and first choice Mirra Andreeva, who also competed in doubles in Riyadh, declared she was not fit to play.
“Ekaterina is a tough opponent, especially at the beginning it’s hard to come back,” said Rybakina, who beat Amanda Anisimova and Iga Swiatek during the week.
“Every match I play I want to win of course, every victory gives you confidence. I hope I can continue.”
Anisimova and Swiatek face off for the second qualification spot from Serena Williams Grour on Wednesday.
The top eight women’s singles and doubles players compete in Saudi Arabia, where £12m ($15.5m) in prize money is up for grabs.
In the Stefanie Grain Group, World Number Onerna Sabalenka, Coco Gauff and Jessica Pegula all remain in semi-final contention.
Sabalenka takes on defending champion Gauff while Pegula faces disqualified Jasmine Paolini in Thursday’s final round robin matches.