WATCH: Novak Djokovic visibly trembles as he struggles physically against Daniil Medvedev in the ATP Finals

Serbian tennis player Novak Djokovic was pushed to the limit in his non-bearing round-robin encounter against Daniil Medvedev in the Nitto ATP Finals. However, just like many times in his career, Djokovic rose above it and pounded out a 6-3, 6-7(5), 7-6(2) win over Medvedev in a lung-busting match that finished in a little over three hours.

The 21-time Grand Slam champion considers Medvedev one of his greatest rivals at present. The match showed that the Russian was indeed a great rival as the Serb was left gasping for air and visibly shaking at a point in the match.

"It was just fatigue from grueling battle. That's all I can say. I mean, there was no illness. There was no particular part of the body that was bothering me. It was just overall physical exhaustion from the rallies and from the length of the play," Novak said.

In a video posted by Tennis TV and picked up by Eurosport, Djokovic covered his face with a towel, his right hand trembling as he was seated on the bench during a changeover.

Asked to explain this specific incident during his post-match press conference, the Serb refused to give details as he did not want to let on too much information to his opponents. Instead, the Serb said that it was just "one of those days."

"Everyone has one of those days where they struggle more physically. For me that was today. I'm very proud to be able to find a way," said the long-time World No. 1.

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"I don't think that a limit exists" - Novak Djokovic

Novak Djokovic sits on the line judge's chair during his round-robin match against Daniil Medvedev in the ATP Finals.

Novak Djokovic once again did the improbable as he rose from the brink to beat fellow stellar hardcourt player Daniil Medvedev for the eighth time in 12 career meetings. While fans and foes are left in utter disbelief and amazement, the current World No. 8 explained it simply in his press conference - for a champion like him, there is no limit.

Asked to explain the second wind that he summons to turn around a match like that and to distinguish the limit between being "destroyed" and becoming a winner, the five-time ATP Finals champion said:

"I don't think that a limit exists. It's really in your head. It's really about perspective and approach and your perception of how you see things in that given moment."

"I think the biggest battle, as I've said before, is always the inner battle. If you manage to find yourself in that optimal state of mind and body as often as you possibly can, I feel you can extract the best and reap the best results for yourself in every moment, every point, every match," Djokovic added.