Nadal, seeking an 18th Grand Slam title, eased to a 6-3, 6-4, 6-2 victory on Arthur Ashe Stadium as the soaring temperatures continued to affect the players.

The 32-year-old Spaniard will next face Russian 27th seed Karen Khachanov, who beat Italy's Lorenzo Sonego 7-5, 6-3, 6-3.

"To win in straight sets is always positive, especially in these conditions," said Nadal.

"We'll be able to see better tennis than what we saw in the first couple of days.

"Playing under these conditions is, first thing, not healthy, and second thing, is good for nobody - not good for the fans, not good for the players. 

"The show is a little bit worse under these conditions."

Serena Williams was also in fine form as she dispatched German world 101 Carina Witthoft 6-2, 6-2 to continue her pursuit of a seventh US Open crown.

Standing in her way in the next round is sister Venus, who overcame Italy's Camila Giorgi 6-4, 7-5.

They will now play each other for the 30th time and for the first time at a Grand Slam since Serena beat Venus in the final of the Australian Open in 2017.

"We'd rather have met later," admitted Serena, who missed last year's US Open after giving birth to daughter Olympia.

Elsewhere, Britain's Andy Murray was knocked out in four sets by Spain's Fernando Verdasco as he continues his recovery from a hip injury.

Verdasco, seeded 31st, recorded a 7-5, 2-6, 6-4, 6-4 win to progress at the expense of the 31-year-old Briton, who was making his firs Grand Slam appearance for 14 months.

The tournament is due to continue tomorrow.