Djokovic stayed unbeaten in the Red Group as he overcame Medvedev 6-3 6-7(5) 7-6(2) in a gruelling clash at the Pala Alpitour in Turin to top the standings.
"Well, it was just fatigue from a gruelling battle. That's all I can say. I mean, there was no illness," Djokovic told reporters.
"Everyone has one of those days where they struggle more physically. For me that was today.
"Of course, the opponent likes seeing you down, and he's trying to dominate in the rallies, he's trying to get the upper hand of the match, which was kind of the situation that was happening today."
The Serbian, who will face American Taylor Fritz in the semi-finals on Saturday, battled back from a break down in the decider to hand Medvedev his third straight defeat after three hours and five minutes.
"Of course, when you're going through physical struggles, it affects the game, it affects how you feel mentally, it affects your body language," Djokovic said.
"I'm very proud to be able to find a way, because that's what we are I think, owing to ourselves and to the team and to the people who come and watch you, is to always try to give your very best in that given moment."
