When infallible champions begin to show symptoms of acute fallibility, then you know that sport has played its cruel hand again.
When sport showers legends like Roger Federer with endless gifts, the world rejoices.
Fans marvel at the mastery of his craft.
The smile, the down-to-earth approach, the formidable forehand, the moves on the court which can be the envy of the best of ballet dancers.
Then sport begins to take away all it has given. It is painfully slow, excruciating to watch, but nobody can do anything to stop them.
Nobody can do a damn to stop this tremendous march of time, crushing under its heavy boots even a player of such calibre.
Then fans applaud, they plead, they just want to watch him turn back time, perhaps for one last time.
Even for sportsmen like the 32-year-old Swiss, winner of 17 Grand Slams, it all comes down to this - timing.