I would be very impressed if Murray managed to injury his glutes in his sleep. I would be a lot less surprise to hear that Murray, who frequently pokes his stiff back, has some sort of nerve irritation.
“I woke up yesterday and was really struggling to walk. I went to a swimming pool nearby and did some exercises and it felt a little better, but I was forced to cancel my practice yesterday. It is just a freak thing and I have no idea how I did it.”