Nadal's year is catching up to him, as the speed on his forehand shows.
First mistake he's made this year is the Laver Cup.
My hypothesis is: "This is Nadal's true level on hardcourts right now - upset prone to anyone who can hurry him on his shots and not allow him time to setup his forehand. US Open was misleading about Nadal's hardcourt level as he was able to win by just playing his clay game."
I could be wrong
, but Shanghai, Paris, Basel, and WTF will pass/fail this hypothesis.