I don't know that any serious fall has begun. Overreaction IMO, but I will say this.
Novak isn't invincible. He's not as good now as he was in 2011 or 2015. Yeah he had the flu, cry me a river, I don't care about that.
Right now, Novak is in the middle of an incredible run where he has a burning desire to finish YE#1 and be the best player in the world for another length of time, however long that is. But it likely won't last as long this time as it did the 2 previous times. That's the way aging works. When you're young it can go on, seemingly forever, but as you get older it gets harder to dominate day in and day out.
Novak is showing the world that he's back now, despite this loss, but what happens after he fulfills his #1 goals this year, and maybe tacks on a WTF title and possibly an AO title in 2019? What motivates him after that? For years we saw it was RG, and then we saw what happened after he fulfilled that dream. We thought Federer's slam record might keep him motivated even after he had won RG, but that didn't work out so well. I'm not trying to be all doom and gloom here. I'm just saying he's lost motivation once before, and he's not exactly young in tennis terms anymore so appreciate this latest run while it's happening.