Djokovic isnt struggling really, its just that the opponent raise their level whenever they play him. Nobody expected Istomin to play as well as he did. Istomin really played his socks off and because of that he got injured in the next round. And in Mexico open, Kyrgios had a great serving session as he hit about 15 aces in the first set while missing only 1 first serve throughout the set. I dont think I have ever seen a player serve that well. Not even Roddick or Isner have done so well with the first serve%.
For me think that Djokovic got what he wanted he taken all Gland slams and think that only one or 2 masters are need to be complete. Think that he is aiming this to achieve and then he can be only player that took all. I really tough that after he won over Murray in season opening that he has recovered from this but think that he is more eager to achieve this goal. if he done this maybe he will go for this first spot again and then when he get this he will retire.