Novak Djokovic returns the ball against Roberto Carballes Baena during the US Open first round on Monday. Photo: IC

Defending champion Novak Djokovic said the timing of the US Open makes it the most unpredictable of the Grand Slams as the Serb made a promising start in his quest for a fourth title in New York.

The world No.1, who has won four of the past five majors, brushed past Spain's Roberto Carballes Baena 6-4, 6-1, 6-4 in the opening round Monday.

He is attempting to become the first back-to-back men's US Open winner since Roger Federer lifted the trophy five years running from 2004 to 2008.

Djokovic, Federer and Rafael Nadal have combined to capture 11 successive and 54 of the last 65 Grand Slams, although success at Flushing Meadows has proved more accessible to those outside the "Big Three" than any other event.

Andy Murray, Stan Wawrinka, Marin Cilic, Juan Martin del Potro and Andy Roddick have all triumphed in New York, whereas only six titles have eluded tennis' most successful trio elsewhere dating back to Federer's 2003 Wimbledon victory.

"Well, maybe what I would point out is this Grand Slam is the last Grand Slam of the year. So some players maybe might have been more exhausted, less freshness in the legs, so to say, than is the case in Australia is the beginning of the season," said Djokovic.

"Because it's a long season, a lot of surfaces that you have to change and it takes a toll on your body... Maybe that's the case why this tournament has provided some different names from the big three that have won the major title. 

"Nevertheless, Del Potro and Cilic are not the outsiders. They have been in top of the men's game, played finals of other slams. They deservedly won this title at least once."

Djokovic famously saved two Federer match points to claim a fourth Wimbledon title in July, his 16th Grand Slam success. Reflecting on that dramatic finale, Djokovic said the key to his remarkable recovery was to live "in the moment" in what amounted to a fight-or-flight situation.

"I think regardless whether you're an athlete or not, I think we all, as human beings, when we face this kind of adversity or major significant moment in our life, it seems like we're going to fail at something that we work so hard at trying to do," he said. 

"Most common reaction is to stress out and just to be unable to get the best out of yourself. So I just try to, as cliche as it sounds, and as simple as it sounds, I just try to be in the moment. 

"I understand that I can't affect anything that is happening off the court or what has happened that got me to that point of being one shot away from losing a match. 

"But I think also the more often that it happens, the more experience you have in knowing how to handle it every next time that you're in that position."