I think that Roger is hiding his chronic back injury very good.
If anyone compared his serve today and let's say 4 years ago ,the results would be obvious.
There is a decrease of the speed of the 1st serve.
I don´t think it´s the back. He´s taken away some of his 1st serve speed, because against Nadal and Djokovic he certainly doesn´t want to start the points with a 2nd serve. So he´s taken some speed off in order to increase his 1st serve %. That´s what I think, at least.