Novak Djokovic opens up on physical struggles as he battles into US Open last 16

The 24-time Grand Slam champion prevailed 6-4, 6-7(4), 6-2, 6-3 against 35th-ranked Norrie in the third round of the US Open after two hours and 49 minutes in Arthur Ashe Stadium.

Djokovic left the court when leading 5-4 in the first set for a medical timeout due to a back issue that was causing him visible discomfort. The Serbian also received treatment early in the second set.

After seeing a 3-1 tiebreak lead slip away to lose the second set, Djokovic was broken in the opening game of the third set, but he responded by winning 12 of the next 16 games to seal his progress to the last 16.

The 38-year-old also struggled physically at times in his first two matches in New York and admitted after his second round win over Zachary Svajda that he "didn't feel that great."

Following his win against Norrie, Djokovic did not hide his concern about his body in a candid interview with ESPN.

"Well, honestly I am concerned. You know, nowadays [I am] more concerned than I have ever been," said the four-time US Open champion.

"Even though I really am meticulous with the care for my body and I put in a lot of hours on and off the court to make sure that my body is fit and recovered well.

"But you know, age is… you can't fight it. So things are different. I have to obviously adjust myself the way I approach things training.

"Sometimes I don't train between matches because with my team when I have this discussion, we think that long term is better and that recovery is more important than hitting some balls in the court.

"So maybe that's going to be the case tomorrow. We have to still discuss that and just focus on recovery and get ready for next one."

The world No 7 also addressed how long he spends on recovery in between matches.

"Actually probably even more when I was younger; it's just in order to keep up with the youngsters I just have to put an extra, extra hour, extra few hours every day," Djokovic explained.

"Whether it's table work, whether it's recovery from using different wellness machines or good sleep, nutrition, obviously all of it, all of it comes into equation and all of it matters when you go deeper in the tournament.

"The young guys, they come in fresh and I come with a half tank empty in a sense and so I'm going to try to change that for this tournament. I don't know if that's going to be possible.

"Obviously, it's very unpredictable how the body is going to respond. But one thing is for sure, I'm always going to give my very best the last drop of energy to make sure that I go as deep as I possibly can."

Djokovic will face world No 144 Jan-Lennard Stuff in the fourth round at Flushing Meadows.