New York - Former champions Roger Federer and Novak Djokovic sought to set up a US Open quarter-final clash on Monday as 2014 finalist Kei Nishikori moved smoothly into the last eight at Flushing Meadows.
Second-seeded Federer, who won the last of his five US Open titles back in 2008, took on 55th-ranked Australian John Millman under the lights on Arthur Ashe Stadium while Djokovic, winner in New York in 2011 and 2015, tackled Joao Sousa.
The signs all pointed to Federer and Djokovic getting past their unseeded opponents and into a seventh meeting on the Ashe stadium.
It would be their earliest meeting in a Grand Slam since 2007, when Federer defeated Djokovic to reach the Australian Open quarters.
Djokovic, however, said he wasn't counting on 44th-ranked Sousa, who advanced past 12th seed Pablo Carreno Busta and No. 17 Lucas Pouille, going quietly.
"He's a fighter, he's a grinder. He will not hand you the victory, you've got to earn it," said Djokovic, who will also have to contend again with sweltering heat and humidity that prompted organizers to reinstate the extreme heat policy that allows a 10-minute break in men's and women's matches that go beyond straight sets.
Nishikori was glad he didn't need more than three sets in the blazing sunshine on Louis Armstrong Stadium.
After racing through the first two sets against Germany's Philipp Kohlschreiber he dropped his serve while serving for the match at 5-4 in the third, but managed to prevail 6-3, 6-2, 7-5.
"Man, really lucky to finish in three sets," Nishikori said. "It was very hot on the court even though it doesn't look too hot it's hit me.
"I think we both struggled a little bit with the heat, but we both played good tennis."
Nishikori could end up fighting for a semi-final berth against Croatian Marin Cilic, who beat him in the 2014 final.
That remains Nishikori's best Grand Slam showing. After reaching a career-high fourth in the world in 2015 he slid down the rankings as a wrist injury sidelined him for six months, keeping him out of last year's US Open and eventually requiring surgery.
Seventh-seeded Cilic, who survived a five-set marathon against Australian teenager Alex de Minaur that went into the early hours of Sunday morning, faced 10th-ranked Belgian David Goffin later Monday.
Cilic is trying to reach the US Open quarter-finals for the fifth time, but he has lost 11 of his last 12 matches against top 10 opponents.
US Open results on Monday, the eighth day of the 2018 tournament at Flushing Meadows (x denotes seeded player):
Kei Nishikori (JPN x21) bt Philipp Kohlschreiber (GER) 6-3, 6-2, 7-5
Madison Keys (USA x14) bt Dominika Cibulkova (SVK x29) 6-1, 6-3