Aryna Sabalenka beat world No 2 Ons Jabeur 3-6 7-6(5) 7-5 at the WTA Finals in Fort Worth, Texas on Monday while Maria Sakkari edged Jessica Pegula to avenge her loss to the American in the Guadalajara final eight days ago.
Sakkari battled past world No 3 Pegula 7-6(6) 7-6(4) to record her first win over a top-five opponent this year and join Sabalenka as the early leaders of their four-player section.
The WTA Finals features eight women split into two groups, with the top two finishers moving onto the semifinals.
The other group features Iga Swiatek, Coco Gauff, Carolina Garcia and Daria Kasatkina.
World No 7 Sabalenka looked set to lose her opening round-robin match as she trailed 2-4 in the decider but managed to close out the thriller in two and a half hours.
"Somehow another miracle happened for me and I was able to win this match," she said.
Tunisian Jabeur, a Wimbledon and US Open finalist this year, could only shake her head after having been two points away from victory while leading 5-3 in the second set tiebreak and one point away from a 5-2 cushion in the third set.
Sabalenka broke at love to start the second set en route to a 4-1 lead and looked poised to cruise into a decider but was suddenly behind 6-5 and had to hold serve to force a tiebreak, where she won four straight points to set up a third set.
In the day's first singles match, Sakkari, who reached the semifinals of last year's Finals, came out more aggressive than her most recent match with Pegula.
The first set lasted 71 minutes - a minute longer than their entire Guadalajara final - and Sakkari secured it on her third set point of the tiebreak with a forehand passing shot.
The Greek raced out to a 3-0 lead in the second set before Pegula battled back, saving a couple of match points and holding serve to force another tiebreak. However, she was unable to send the match to a deciding set.
"It's never easy against Jess. She's an amazing player, she's very confident right now," world No 5 Sakkari said. "I knew it was going to be extremely tough but I fought hard and trusted my game."