The two lower-seeded players pulled off upsets Monday as the WTA Finals got underway in Fort Worth, Texas.
In the initial matches of the Nancy Richey Group, No. 5 Maria Sakkari of Greece topped No. 3 Jessica Pegula of the United States 7-6 (6), 7-6 (4), and No. 7 Aryna Sabalenka of Belarus rallied past No. 2 Ons Jabeur of Tunisia 3-6, 7-6 (5), 7-5.
The season's eight top players were divided into the Nancy Richey Group and the Tracy Austin Group for round-robin play. The winner of each group will play the runner-up from the opposite group in the semifinals, with the victor in those two matches heading to the final on Nov. 7.
The prize pool is $5 million, and an unbeaten champion would earn $1.68 million.
The Tracy Austin Group kicks off on Tuesday with No. 1 Iga Swiatek of Poland opposing No. 8 Daria Kasatkina of Russia before No. 4 Coco Gauff of the United States squares off against No. 6 Caroline Garcia of France.
Sakkari served for the first set at 6-5, Pegula broke to force a tiebreaker. After taking the first three tiebreaker points, Sakkari moved to a 5-2 lead and eventually sealed the set on her third set point.
Pegula fell behind 3-0 in the second set but pulled level at 3-3. As in the first set, Sakkari had a big chance at the 12th game, but Pegula staved off two match points to set up another tiebreaker. Sakkari claimed the final four points to clinch the match.
"I was very aggressive," Sakkari said. "If you're passive, Jess is just going to make you run. She's going to hit the ball very hard, and it's over. ...
"I don't think it ever crossed her mind that it's going to be an easy match because she beat me a week ago. I never felt like she underestimated me today."
Sabalenka lost serve to open the match against Jabeur, who went on to control the opening set. Sabalenka bounced back to take a 4-1 lead in the second before Jabeur forced a tiebreaker that Sabalenka won to level the match.
The final set saw each player drop serve once before Sabalenka got the decisive break in the final game.
"I was just like, 'Keep trying, keep fighting and make sure that if she wants to win this match, she has to do something great, not like an easy win,'" Sabalenka said "And I think that's why I put a little bit more pressure on her and in the key moments, I was able to win it."
WTA Finals group standings
Nancy Richey Group
T1. Maria Sakkari, 1-0
T1. Aryna Sabalenka, 1-0
T3. Ons Jabeur, 0-1
T3. Jessica Pegula, 0-1
Tracy Austin Group
T1. Iga Swiatek, 0-0
T1. Coco Gauff, 0-0
T1. Caroline Garcia, 0-0
T1. Daria Kasatkina, 0-0
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