Why are Nadal and Djokovic the top players in the world? I would suggest their ability to play defense, retrieve, get every ball back, and then transition from defense to offense in one shot is a major factor. So many times I have seen both of these guys hit passing shots and flat-out winners from 10 feet behind the baseline.
The edge they are getting on the field is not luck or necessarily all genetics. Nadal and Djokovic have put in the work to be able to play incredible defense. Check out this article from the Washington Post prior to their Wimbledon final. It highlights why these guys have separated from the field.
Nadal Djokovic New Breed Washington Post
Let me remind you that training and preparation are foundational factors in the development of a highly confident tennis player that is performing at a championship level. So, while this piece focuses on the physical aspects of performance and how it has elevated the level of play, do not overlook the confidence that is gained from having the ability to retrieve and hit with power and accuracy ball after ball after ball. Your shot tolerance can be real high if you are in great shape!