It is worth remembering that what you are watching is not normal.
It is not normal that a 39-year-old should beat an opponent 14 years his junior in five sets in the quarter-finals of a Grand Slam.
It is not normal that after five hours and 15 minutes, he is still chasing down every single shot that is pounded towards his racquet.
It is not normal that he is gliding and sliding and then charging around the baseline, running to the net to meet drop shots, and still sending down second serves at 122 mph.
But then, this is Novak Djokovic.