Djokovic's mental strength is actually a WEAPON for him, he not only doesn't choke on important points but players much better during them. Nadal on the other hand is strong in the way that he never gives less than 100%. He tends not to get the lapses in concentration that the other 3 get sometimes. Fed and Murray are less commendable, they can hold it together but there is nothing special about their mental strength.
"To be honest, I think bananas are a pathetic fruit." ~Andy Murray