What displayed itself is what won 40-15.
It was a message to Nadal. Djokovic just used media to make sure Nadal see it and get reminded several times before SF.
Relief due to stress? Watch him after doing it. No sign of relief. No sign of disscomfort for doing it. Ask an experienced psychiatrist.
Surely, Nadal is heavy favorite to win, 70/30 or whatever. But Nadal's level dropped in QF (for how long?) and It was not due to Schwartzman's level went ATG. The same drop in level by Nadal in SF will end match differently.
That remainder was Djokovic's message to Nadal and Djokovic only made sure Nadal has it on his mind when they enter the court today.
Simple.