Jessica Pegula beat world number one Aryna Sabalenka in straight sets to move into the semi-finals of the WTA Finals in Cancun, Mexico
Pegula, 29, clinched victory on her seventh match point, sealing a 6-4 6-3 victory over the Belarusian, who made 33 unforced errors.
Elena Rybakina defeated Maria Sakkari 6-0 6-7 (4-7) 7-6 (7-2).
She will now play Sabalenka on Thursday evening to determine the second qualifier from the group.
American, Pegula who had defeated Rybakina on Sunday, has now won her past five matches against top-10 players and said she is feeling “more comfortable” against the top players this year.
“I don’t think it feels like such a big deal,” she said.
“Last year when I made top five, I think I psyched myself out a little bit. You have thoughts in your head – do I really belong at that level?”
In contrast Sabalenka, rarely looked at ease on the newly constructed Estadio Paradisus and described the playing surface as being “like leather”.
The 25-year-old is one of several players who have complained about the condition of the court at this week’s tournament.
“You never know what’s going to happen, whether the bounce is going to help you or work against you,” Sabalenka said.
“I guess today was a day where it didn’t work well for me. But it’s OK I still have a chance to pass the group, so I’m trying to stay strong. I’ll do some recovery and just try to analyse what happened. I’ll do everything I can in the next match.”
Wednesday’s Group B matches see Wimbledon champion Marketa Vondrousova take on Ons Jabeur, while US Open winner Coco Gauff faces Iga Swiatek.