CAMELOT FANTASIES

General, he can play doubles into his 50's and 60's if he wants, same with poker, golf, and other relaxing sports.

He'll probably have a billion in the bank by then.

His time for playing singles tennis at the highest level is drawing to a close.

The curtain may have already set if the hard work won't be there.

He just lost in his opening round in Shanghai to Viktor Troicki of all people in straight sets, 6-3, 7-6(3). First win ever for Troicki over Nadal.  He used the same formula as many have lately to win.  Stay aggressive on the court and wait for the inevitable short ball and go for it.

And his rabid blind toothless fans tell everyone to stop being negative and to shut up. 
Burying one's head in the sand won't help.
Facts are facts, objective fans face reality, his fall cannot be ignored.  Sudden falls have happened before, to players even younger.  Wilander won 3 majors in a year at age 24, and nothing more thereafter. McEnroe had his greatest season at age 25, maybe the greatest in history in 1984 (82-3 and 2 majors) and never won another major.

As tennis fans we can only suggest the best course of action.  I don't even know if being a coach would help at this point.

The worry one might realistically have is that if he insists on playing with this level of fitness, that he might take himself out of the game with injury.
His mind is simply trying to make promises his body can't keep.

Respectfully,
masterclass