Day two of the US Open delivered the tournament’s first big stunner. Renata Zarazúa of Mexico defeated No. 6 seed Madison Keys 6-7(10), 7-6(3), 7-5 in a major first-round upset at Arthur Ashe Stadium Monday afternoon.

NEW YORK — Australian Open champion Madison Keys is out of the U.S. Open in the first round. Renata Zarazúa, the world No. 82 from Mexico, came from a set and 3-0 down to stun the No. 6 seed 6-7 (10), 7-6 (3), 7-5 in bright sunlight on Arthur Ashe Stadium. Zarazúa also became the first Mexican woman to ever beat a top-10 opponent at the U.S. Open.

Pretty much from the get-go at the U.S. Open on Monday, Madison Keys could tell she wasn’t hitting the ball well or feeling very much at all like the self-confident player who claimed her first Grand Slam title at the Australian Open in January.