Anhelina Kalinina was down and almost out before clawing her way back to end former world No.1 Caroline Wozniacki's participation at the Miami Open.
The Ukrainian saved match point and won the last five games of the second set before taking the third in a 5-7 7-5 6-4 victory in the second round.
Wozniacki, who won the Australian Open in 2018 and retired from tennis at the same tournament two years later, returned to the sport last year. She had two children during her time away from the game.
Wozniacki reached the quarter-finals in Indian Wells this month but retired from the match against top-ranked Iga Swiatek in the second set because of a right foot issue.
Beatriz Haddad Maia also overcame a slow start in hot and humid conditions on the hard courts, rallying to beat Diane Parry 3-6 6-1 6-4. It was the Brazilian's first career victory in three tries against Parry, who won both previous matches on clay.
In other women's matches, Katie Boulter advanced when Brenda Fruhvirtova retired from their match while trailing 7-6 (7-5) 1-0, Maria Sakkari beat Yuan Yue 6-2 6-2, Taylor Townsend defeated Elise Mertens 6-2 6-2 and Anna Kalinskaya beat Wang Xiyu 6-2 6-2.
The 32 seeded players were given first-round byes. Aryna Sabalenka, the No.2 seed in the women's draw, is expected to play her opening match against Paula Badosa on Friday.