If anything, Murray can help Novak get in the best cardio shape of his life. When Novak is confident that he can outlast anyone on the court, he would be better able to focus and execute his game plan.
Oh all my French and French-speaking Swiss friends say Djokovic speaks better French than Federer. In fact, they tell me they are quite disappointed at Fed’s mediocrity in the French language.
If it is less than a year old, contact the retailer you bought the racquet from and file for warranty replacement. I would not take the buttcap off. It would only give the manufacturer an excuse to void its warranty.
So far, the only players who have better endurance than Djokovic in the best of 5 are Alcaraz, Sinner, and Medvedev.
The 37 year old is still in better shape than the 99% of the field.