This tweet from Zemek says it quite well. Novak had a rough day after a superb year. Losses happen.
Consider this as well.
If you've ever been jammed into an immediate follow up match by a
tournament director while your opponent is rested, you know the issue. I've had it happen to me twice in my youth. In Novak's case, it is apparent that previous long matches took their toll that made full recovery in the time available very unlikely.
He was spent at the beginning. His legs looked dead. He tried various changes such as taking the net to make up for it, but to no avail. It wasn't going to be his day.
This happens to the best and has over the history of the sport. What I'm amazed at is the level of visceral hatred for Novak and the gloating that he lost. His playing record still indicates that he's one of the best ever and no matter the chatter, he's at least the equal of the other members of the Big 3.
The Calendar year Grand Slam is a unique measure for men's tennis among many metrics and maybe it's meant to be so rare that only two men will hold it. It allows Laver to stand alone for his two CYGS making his lower numbers in other categories less reliable indicators. We know he was prevented from slam competition during much of his prime when he turned pro as were many other big names. Here is another take on what Med did. ATP Article.
I think all of the strategy available to Novak wouldn't have made up for lethargy. He was below par and not admitting that means you weren't watching.
Novak isn't done for sure. I expect him to win more events and more slams.