World number one Novak Djokovic has continued his flawless march at the US Open with a comfortable straight-sets win over Portuguese qualifier Joao Sousa in the third round.

Djokovic made a blistering start to claim the opening set 6-0 in just 28 minutes, with Sousa bewildered at the other end of the court to concede three successive breaks of serve.

Sousa eventually claimed a game at the eighth time of asking to avoid any thoughts of a humiliating triple-bagel defeat, but Djokovic soon responded as he rattled off the second set with minimum fuss.

The Serb looked to be sauntering through to another bagel in the third set, but a plucky Sousa finally made a dent in his opponent's serve to earn a break his efforts merited.

That break only served to anger Djokovic, though, with the Serb claiming four successive points on Sousa's serve to wrap up a 6-0 6-2 6-2 win and book a place in the last 16.