Novak Djokovic said he battled with dizziness early in his three-set win over Andreas Seppi at the US Open.

The top seed advanced 6-3 7-5 7-5 on Friday, but conceded the sweltering New York conditions - the temperature has peaked in the low-to-mid 30s in degrees Celsius in the first week of the major.

The heat influenced the make-up of Djokovic's next battle too, with 14th seed David Goffin retiring due to the conditions - Roberto Bautista Agut gaining passage as a result.

"Yes [I was feeling dizzy], at the beginning, first set, I was struggling. I managed to break through and felt better after that," Djokovic told a news conference.

Overall, Djokovic was content with the result - despite failing to serve out the match at the first attempt, and facing break points the second time of asking before eventually responding.

"Satisfied, but you can always do better. We're all perfectionists in a way," the Serbian said.

Interestingly, Djokovic said he was happy he has shaken the habit which saw him bounce the ball an unnatural amount of times before serving - especially prior to key points.

"I got a few warnings, time violations, with ball bouncing," he replied, when asked about his previous habit. 

"I was aware of the fact that it was annoying also the opponents. I wasn't doing it on purpose obviously. 

"As the situation is more important in a match and you're facing break points, I guess the more bounces I would get. It was a habit that I didn't want to keep, so I had to work on it obviously - mentally mostly, because in the practice sessions I didn't do that much. It was more on the match. 

"I'm glad I got rid of it. I occasionally bounce the ball a bit more than the average number, but it's still fine."