I have only seen highlights so far, too, so it's hard to tell, but it did seem like Novak kind of folded, given what we know about Zverev.
I think you are comparing apples to oranges in terms of mentally tough Serena v. mentally tough Novak. Serena, pre-baby, was one of the toughest competitors in the game, men's or women's. In 2015, and 2 matches away from the real GS, Serena buckled. I do not think that Novak has been generally as strong, mentally, as Serena. You want to say that, over the course of his career, since he has challenged Fedal, he's been tough. But that's where I say false comparison. It's not a fair comparison to make it for Novak about across his career, and for Serena about a specific moment. Djokovic has actually wilted under certain types of direct pressure. Which has been less true of Serena, until post-baby, and also chasing the elusive 25th Major.
Novak fell apart basically for 2 years after finally completely the career slam/Nole slam. He couldn't pull off the win v. Wawrinka in 2015, after he managed to beat Nadal at RG. He was pretty much utter crap at the end of last year, after a spectacular start. When he's on, and confident, he does seem unbeatable, but when he's off, please don't tell me he's as tough as Serena, for most of her career.