Madison Keys becomes only the second woman, after Angelique Kerber, to reach the quarter-finals or better at three of the four Grand Slams this year.
Madison Keys eased into the quarter-finals of the US Open with a 6-1, 6-3 victory over Slovakia's Dominika Cibulkova on Monday.
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.
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.
Next up for Keys, who was routed by Sloane Stephens in last year's final, is the winner of the evening clash between five-times Grand Slam champion Maria Sharapova and Spanish 30th seed Carla Suarez Navarro.
Nishikori advances to quarters
Kei Nishikori will be hoping to make another deep run at his most successful Grand Slam event after he easily outclassed Philipp Kohlschreiber 6-3, 6-2, 7-5 to reach the quarter-finals of the US Open on another blazingly hot day on Monday.
The Japanese 21st seed, a finalist at Flushing Meadows in 2014 and a semi-finalist in 2016, fired 29 winners to end the run of Kohlschreiber, who had beaten fourth seed Alexander Zverev in the previous round.
Nishikori appeared to be racing towards an easy win before Kohlschreiber unexpectedly broke the 28-year-old to level the third set at 5-5.
But that only delayed the inevitable as Nishikori kept his focus to seal the win and extend his perfect record against the German to 3-0.
"(I was) really lucky to finish in three sets -- it was really hot on the court," a sweat-soaked Nishikori said in an on-court interview. "I think we both struggled a little bit with the heat."
Nishikori will next face either seventh seed Marin Cilic, which will be a rematch of the 2014 final, or Belgian 10th seed David Goffin.