Novak Djokovic indicated a spectator he was seen arguing with during a practice session helped motivate him for his US Open third-round victory over Denis Kudla.

Djokovic brushed off concerns over his left shoulder, on which he required treatment in his second-round match against Juan Ignacio Londero, to claim a 6-3 6-4 6-2 victory in Arthur Ashe Stadium and book a meeting with Stan Wawrinka.

The world number one and defending champion delayed his practice by two hours ahead of the contest, and that session was marked by a confrontation with a fan in which the Serbian appeared to say "I'll come find you".

Noticeably fired up during the match, Djokovic also took issue with members of the crowd who cheered a double fault, firing back at them after he saved a break point.

Speaking in his post-match news conference, Djokovic was reticent to give too much away about the practice incident, which he initially described as "just a little chat".

Asked about the "come find you" remarks, Djokovic replied: "To have a drink. I liked the guy. I'm going to buy him a drink."

Pressed on what was said to him, he added: "We'll keep it between us. But he definitely helped me. He doesn't even know, but he did help me.

"As I said, I'm not going to talk about it. I think he did me a favour. Even maybe he didn't want to do me a favour, he did me a favour, big favour."

Djokovic conceded to being fired up by the behaviour of some of the crowd during the match, saying: "Night sessions, New York, crowd gets into it. A couple guys that had a couple of drinks more than I guess they were supposed to. But it was all good after."

Despite declaring himself "almost pain free", Djokovic was reluctant to give details on what treatments he has had since the Londero match.

"As I said, I would appreciate if you respect me not talking about it in details," he responded when asked if he had been given a pain-killing injection.

"I mean, I understand you guys want to know. I made a decision not to get into details, not to speak about it. Please understand me.

"I'm very glad with the way it went. I am able to play. That for me is a huge blessing today because it was probably the complete opposite two days ago.

"I did not practice yesterday, that's true. I did a lot of things in the last few days to be able to play."