Mexico's Renata Zarazua pulled off a stunning upset by beating sixth seed Madison Keys while twice major winner Barbora Krejcikova knocked out promising Canadian teenager Victoria Mboko, as the U.S. Open first round continued on Monday.

Canada’s Victoria Mboko’s U.S. Open debut ended early with a 6-3, 6-2 loss to Barbora Krejcikova of Czechia in a women’s first-round singles match Monday. Mboko was playing her first match since claiming the National Bank Open title in Montreal earlier this month.