Djokovic had a similar thing last year when he had some sort of minor injury before the ATP Finals but played the event and played it well. He experienced some minor physical issues in this match as well but played it down because he doesn't want to make excuses for losing a match:
“I don’t want to talk about that, because I know that you guys might take that as, you know, excuse or something like this why I lost. No, everything was okay. I mean, it’s normal that when you play day after day and you have some tightness here and there, but I didn’t have an injury. I fought till the last moment. So it wasn’t to be for me today.”
Teams confirmed for World Tennis League including Swiatek, Bouchard, Zverev, Thiem, Djokovic, Kyrgios and Andreescu
“Of course I’m disappointed with the loss, but I was very close. It was just very few points that decided a winner. But the level of tennis that I’m playing is high, and I like my chances. You know, of course every match is like finals there. You know, there is no easy matches. You’ve got to be fit, fresh. Good thing is that you have a day between every match in the group stage, so you have time to recover between each match, which is good."