After Djokovic injury and his subsequent comeback, more likely he is somewhat rejuvenated that's why he looks fresh as compare to Nadal which is just a year older than him. I agree that he can overtake Federer because as I have said, he found second wind and the two Nadal and Federer, can't cope up unless they rested their body and carefully choosing tournaments they want to enter. So there's no question that he is really back to reclaim the top spot again and this is bad news. LOL. Yeah, congrats to Djokovic and Osaka as well, for being the first Japanese player to win a US Open or any other majors for that matter.
I also think that maybe Djokovic will soon start to pick less tournaments like how Federer and Nadal have been doing the past couple of years. If he does decide to do this then I think it makes his chances at becoming the top Grand Slam winner a lot higher then if he plays is many tournaments through out the year. It will be interesting to see if Nadal will take a rest now with his knee and come back next year fresh and ready to go for the Australian Open. Federer I think can play in a few more tournaments this year but not sure about Nadal.