Sakkari, Badosa victorious at WTA Finals

Sabalenka, who won titles in Abu Dhabi and Madrid this season, is the top seed in Mexico because Australian world No.1 Ash Barty of Australia decided not to defend her WTA Finals title due to continuing COVID-19 travel and quarantine restrictions.

Barty had to undergo two weeks of quarantine when she returned to her Brisbane home in September after the US Open and did not want to do the same again travelling to Mexico.

Badosa is having a breakthrough season after starting the year ranked 70th.

The Spaniard will play her next match against Maria Sakkari, who used her strong first serve to extend her dominance over former French Open champion Iga Swiatek with a 6-2 6-4 win.

The 26-year-old Sakkari, who is also having a breakout year, also beat Swiatek in straight sets in the French Open quarter-finals and Ostrava semi-finals this year.

Sakkari started the year ranked 21st and although she did not win a title this season, she reached the final in Ostrava and the semi-finals at the U.S. Open, Roland Garros and Miami to become the first Greek woman to qualify for the WTA Finals.

Swiatek, who won titles in Adelaide and Rome in 2021, is also making her debut at the season-ending tournament.

"Yeah, I think it was a very solid match from my side," Sakkari said.

"Obviously my serve really helped my game.

"I felt quite good with the altitude. I could control my shots pretty well. I think every day I'll feel even better."

All of the players are having to adjust their game to the nearly 1,500 metre altitude of Guadalajara, Mexico's second-largest city.

The $5 million ($A6.9m) tournament, which was cancelled last year because of the COVID-19 pandemic, features a round-robin group stage before the semi-finals and final.

The WTA Finals are being played in Guadalajara for this year only, with the event scheduled to return to Shenzhen, China, in 2022.