Spain's Paula Badosa fired 10 aces in a 7-6 (7/4), 6-4 round-robin victory over Maria Sakkari Saturday and secured a semi-final spot at the season-ending WTA Finals.
With one round-robin match remaining, Badosa was assured of top spot in her group when top-seeded Aryna Sabalenka of Belarus defeated Iga Swiatek 2-6, 6-2, 7-5.
Poland's Swiatek was eliminated from semi-final contention, and Monday's match between Sabalenka and Sakkari will determine the second semi-finalist from the group.
Badosa, whose late season run of form included her biggest title to date at the pandemic-delayed hardcourt tournament in Indian Wells in October, had already posted a convincing victory over Sabalenka to open group play.
Saturday's clash between Badosa and Sakkari, the first Greek to qualify for the finals, was the first meeting between the two.
Badosa needed just over two hours to stretch her current WTA winning streak to eight matches. After eking out the first-set tiebreaker, she grabbed an early break on the way to a 3-1 lead in the second set.
Unable to capitalize on a string of break chances in the fifth game, Badosa was broken herself as Sakkari knotted the set at 4-4.
Badosa responded immediately with a break for 5-4 and a chance to serve out the match, sealing it with a backhand winner on her third match point.
"I think it was quite a tough match," said Badosa, who was assured of a semi-final spot when Sabalenka took the second set off Swiatek in their later match.
"I served well, I fight for every point. I think I stayed aggressive. I was moving well.
"When you win these kind of matches, I think you have to do a little bit of everything well. I think the key was a little bit in the important moments, I was quite brave and I played very good."
Sabalenka overcame 16 double faults to keep her hopes alive in the elite season finale, which was moved to Guadalajara from Shenzhen, China, because of the coronavirus pandemic.
She surrendered the opening set with a double fault on set point, but stepped up her service game in a second set in which she broke Swiatek twice without dropping her own serve -- levelling the match with a service winner on set point.
After breaking Swiatek for a 2-1 lead in the third set Sabalenka was broken back on another double fault.
But Swiatek couldn't capitalize on Sabalenka's inconsistency. Sabalenka broke for a 6-5 lead and with a chance to serve for the match she didn't falter, closing it out with a service winner.