Nadal is the heavy favourite to win his 11th French Open title at Roland Garros and he started his campaign with a straight-sets victory against Simone Bolelli.
Next up for the world No 1 is Guido Pella - an opponent he comfortably beat in their only previous encounter.
The King of Clay is not expected to drop a set until he reaches the latter stages of the competition.
And McEnroe says Nadal’s fighting spirit is his secret weapon that most players on the tour cannot match.