Fourth-seeded Sakkari took advantage of Sabalenka’s 19 double-faults in the longest match so far at the tournament in Mexico, lasting 2 hours and 47 minutes.
“I think it was a roller coaster match from both of us. I was up a break, then she was up a break. It was just a matter of who took the chances,” Sakkari said. “I think at the end I just played with my heart and fought well, just turned things around.”
The 26-year-old Sakkari started the year ranked 21st. After reaching the semifinals at Roland Garros and the U.S. Open, she became the first Greek woman to qualify for the WTA’s season-ending tournament. Paula Badosa will play Garbiñe Muguruza in Tuesday’s other semifinal match.
“My season is not finished yet, but this tournament has been very successful after today’s win,” Sakkari said. “I’m just very, very happy that I still play with my heart even though it’s been 11 months since we started this season.”
Sabalenka, who was the No. 1 seed in Mexico because top-ranked Ash Barty of Australia decided not to defend her WTA Finals title, kept in the contest despite struggling with her serve.
Sakkari fell behind 5-3 but saved a set point before breaking Sabalenka’s serve to force a tiebreaker, which she dominated with three aces.
Sabalenka rallied in the second but still missed three set points before pulling level with a tight win in the tiebreaker.
The 23-year-old Belarus star then went ahead 3-1 in the third but self-destructed with double faults, and Sakkari won five straight games to secure the victory.
“I just couldn’t find my rhythm. The whole match I was, like, really struggling with my serve,” Sabalenka said. “On some moment I found it, and on some moments I couldn’t do anything with my serve.
“It was a tough match. Just crazy how bad I was serving I would say on the last matches.”
Earlier, Iga Swiatek spoiled Badosa’s birthday with a 7-5, 6-4 win, but it didn’t change the outcome of the group.
The 20-year-old Swiatek, the youngest player in the eight-woman field, rallied from a break down in each set.
“I’m pretty happy that I had a chance to show my tennis finally, overcome all these factors around, just play the game,” Swiatek said. “For sure this tournament was special. Playing three days with such high-quality players, it’s hard. I hope my next time is going to be a little bit more lucky for me.”
Badosa, who turned 24 on Monday, had already qualified first in their group with her opening two wins and might already have been looking ahead to her semifinal against fellow Spaniard Muguruza.
“It’s an important one,” Badosa said. “For me it was already a dream to be here, so you can imagine how I feel to play the semifinals and against her.”
Badosa won the first tour-level titles of her career this year in Belgrade and Indian Wells, and is among six players to have made their top-10 debut this season.