Sounds like a little bit of burnout; no cause for alarm.
Novak Djokovic: 'After the French Open I lost enjoyment for tennis'
Gatto Luigi - 21-09-2016
Tennis - Regarding his toe injury, he said, 'I'm doing well'
http://www.tennisworldusa.org/news/news/Novak_Djokovic/36500/novak-djokovic-after-the-french-open-i-lost-enjoyment-for-tennis-/
In most cases, winning helps to win even more, but for Novak Djokovic it wasn't the case. The world no. 1 won almost everything in the first half of 2016 including completing the career Grand Slam at the French Open, but thereafter he was not the same player as before.
'To be honest I am looking forward to finding the freshness and peace of mind of enjoying sport and enjoying being on the tennis court,' Djokovic said to Sky Sports about the remainder of his season, where he will play Beijing, Shanghai, Paris Masters and the ATP World Tour Finals. Somehow I lost it along the way after Paris and the French Open. I felt that over the last couple of months it was difficult for me to re-engage myself.'
However, Djokovic added that it had been a positive season despite his defeats. 'It’s been another great season and it is not over. Now I am looking forward to Asia and competing again, hopefully healthy.',
Asked how he has been doing with regard to the toe injury that he'd suffered during the US Open final against Stan Wawrinka, Djokovic didn't elaborate much, but said he was fine. 'I am doing well, thanks for asking me. The Grand Slams in 2017? No, it's too early to speak about next year.'