Sakkari, seeded fourth, prevailed 6-1 5-7 6-4 on Friday night and will take on Marie Bouzkova, who advanced after Russia's Anna Kalinskaya retired with the score 5-2 in favour of the Czech in the first set.
Sakkari will play in the WTA Finals in Fort Worth, Texas, for the second straight year. The 27-year-old from Greece also advanced to the semi-finals in Guadalajara a year ago.
"As bad as my season, like, after Indian Wells was, I still just gave myself this chance to finish the year strong. I think that's the best gift I can give to myself, just making it to Fort Worth," said Sakkari.
Top-ranked Iga Swiatek and Ons Jabeur secured their WTA Finals berths with their efforts at the US Open, and Jessica Pegula arrived in Mexico after punching her ticket last week in San Diego.
Others who qualified included Coco Gauff, Caroline Garcia, Aryna Sabalenka and Daria Kasatkina.
Gauff, who secured her place with two wins this week, lost to two-time Australian Open champion Viktoria Azarenka 7-6 (7-2) 4-6 6-3 in their quarter-final on Friday.
The 33-year-old Azarenka, from Belarus, reached her first semi-final since last October, when she advanced to the final at Indian Wells.
Azarenka, a former No.1-ranked player, will face off against third-seeded Jessica Pegula, who cruised to a 6-2 6-2 win over former U.S. Open champion Sloane Stephens.
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