Maria Sharapova beaten by birthday girl Carla Suarez Navarro in New York

Sharapova, the 2006 champion, had never lost a night session match in New York prior to her first Grand Slam meeting with Suarez Navarro.

"Thank you so much guys," Suarez Navarro said courtside after being wished happy birthday by the crowd. "It's the first time I've played this year at night. I'm very happy because I played a good match."

Five-times Grand Slam champion Sharapova paid the price for 38 unforced errors and won less than half of her first serve points while committing eight double faults. She said:

Suarez Navarro will next face Madison Keys, who earlier eased into the last eight with a 6-1 6-3 victory over Slovakia's Dominika Cibulkova.

The 14th-seeded American, looking to go one better than her runner-up finish at Flushing Meadows last year, relied on both her dangerous serve and powerful groundstrokes to deny Cibulkova any chance of an upset win.

Cibulkova, who arrived on the Arthur Ashe Stadium court brimming with confidence after toppling Wimbledon champion Angelique Kerber in the previous round, countered with a similarly aggressive game to little avail.

With the US Open's heat advisory put back into effect, the pair traded punishing groundstrokes under the blazing sun and it was Cibulkova who was often pinned well behind the baseline and at the mercy of her opponent.

Keys has been solid in New York, having dropped a single set through four matches, and turned it up a notch against her 29th-seeded opponent as she lost only 10 points on serve during a 76-minute match in which she was broken once.

"The biggest thing is just being able to push her back off of the baseline where she likes to hold her ground and just be able to dictate points like that," Keys told reporters.

Cibulkova, who had not reached this stage of the year's final Grand Slam since reaching the last eight in 2010, pulled ahead 3-2 in the second set but Keys stormed home from there and clinched the win on her second match point when the Slovak sent a forehand wide.

With the win, Keys improved to 5-0 lifetime against Cibulkova and becomes only the second woman, after Kerber, to reach the quarter-finals or better at three of the four Grand Slams this year.