The 37-year-old Swiss star, seeking a 21st Grand Slam title, hit a superb forehand winner around the post in a 6-4, 6-1, 7-5 triumph at Flushing Meadows.

Federer was two sets up at the time of the shot, which he described afterwards as "special".

"I definitely think it was a special one, no doubt about it," said the second seed, who is chasing a first US Open crown since 2008.

"You don't get an opportunity to hit around the net post very often."

"You can't train for that.

"The net is out further and the court is more narrow, so for a shot like this to happen in a practice, you will be running into a fence and you will hit it into the net."

"These shots can only really happen on a big court."

Federer will next face another Australian in John Millman, who overcame Kazakhstan's Mikhail Kukushkin 6-4, 4-6, 6-1, 6-3.

Serbian Novak Djokovic, the 2015 US open winner, is also through after he dominated Frenchman Richard Gasquet on his way to a comfortable 6-2, 6-3, 6-3.

If Djokovic gets past unseeded Joao Sousa, Djokovic could face Federer in the quarter-finals.

Croatian Marin Čilić, who won the tournament in 2014, progressed to round four with a battling 4-6, 3-6, 6-3, 6-4, 7-5 win over Australian Alex De Minaur in a match which finished at 2am local time.

In the women's draw, defending champion Sloane Stephens is the only remaining top-six seed after Wimbledon champion Angelique Kerber of Germany and the Czech Republic's Petra Kvitová both suffered third-round exits.

Kerber was beaten 3-6, 6-3, 6-3 by Slovak 29th seed Dominika Cibulkova, while Belarusian Aryna Sabalenka beat Kvitová 7-5, 6-1.