Spanish world number one Nadal, 32, defeated the Canadian 6-3 6-4 6-2 in exactly two hours.

The 17-time Grand Slam champion will play Russian 27th seed Karen Khachanov in the next round at Flushing Meadows.

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

In the sweltering New York heat, three-time US Open champion Nadal broke early to go 3-0 up before wrapping up the first set in 35 minutes.

Pospisil put up more of a challenge in the second set, leading briefly after breaking serve before Nadal broke back immediately and won four games in a row.

He proved far too strong for Pospisil in the third, appearing considerably fresher than he had during his first-round match with fellow Spaniard David Ferrer, who was forced to retire.

On Wednesday, US Open organisers were forced to impose "extreme heat" rules - although rain is expected in New York later in the week.

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

"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."