Canada's Bianca Andreescu upset Russia's Liudmila Samsonova 7-6 (1), 4-6, 6-2 in the first round of the San Diego Open on Monday.
Samsonova, ranked 23rd in the world, had won three of her past four tournaments entering the WTA Tour 500 event.
Andreescu, ranked 57th, was playing for the first time since losing in the third round of the U.S. Open early last month.
Andreescu, from Misssissauga, Ont., will next face the winner of a match between two 18-year-old Americans -- No. 6 seed Coco Gauff and Robin Montgomery.
Leylah Fernandez of Laval, Que., faces No. 8 seed Daria Kasatkina of Russia in a first-round match on Tuesday.
