Dhaka, Aug. 27 -- Mexico's Renata Zarazua pulled off a stunning upset by beating sixth seed Madison Keys while American Frances Tiafoe launched his latest bid to end the American men's drought at the US Open with a confident win over Japan's Yoshihito Nishioka.

Keys finally claimed her maiden major at the Australian Open this year but produced 89 unforced errors in the first match of the day on Arthur Ashe Stadium as she became the highest seed in the tournament to fall in a 6-7(10), 7-6(3), 7-5 defeat.

Zarazua, the only Mexican player in the US Open main draw, saved seven break points across the match for her first win over a top-10 player in a gruelling, three-hour and 10-minute affair.

"Coming into the match I was like almost crying...