At 38 years of age, Federer still heads into a Grand Slam as one of the favourites.
He rolled back the years at Wimbledon last month, reaching his 12th final at the British slam after defeating Nadal in the semi-finals.
Federer and Djokovic then went toe-to-toe in an all-time classic as the match went to a deciding set where the world No 3 had two championship points.
But the Serbian saved both points before going on to win his fifth Wimbledon title.
Both Federer and Djokovic took an extended break before returning to action at the Cincinnati Masters, with the former suffering a surprise loss to Andrey Rublev while the world No 1 reached the semi-finals before being defeated by eventual champion Daniil Medvedev.
Nadal meanwhile stormed to victory at the Rogers Cup in his one and only tournament to prepare for the US Open.
And Roddick believes the two top ranked ATP players in the world are the “clear” favourites to win the US Open.
But the 2003 US Open champion hasn’t written Federer off completely.
“The two clear favourites are Novak and Rafa,” he told the New York Post.
“Roger is low on match-practice.
“And that loss was tough at Wimbledon.
“He’s going to need the crowd and be lifted.
“Fortunately, I’ve never seen a guy more beloved than Roger.
“He’s going to be searching for form because he doesn’t have a lot of matches.
“If he can get through the first week clean, he’ll have a shot.”
Federer faces Sumit Nagal in the first round, with the match scheduled to take place following the conclusion of Serena Williams against Maria Sharapova on Monday evening.