#4 Has a better career winning percentage
Tennis cannot be mastered by mastering a single surface; all world class players have to prove their mettle on all surfaces. While they may not be the best everywhere, they are still expected to have respectable numbers playing on their least preferred surfaces.
Federer has played a total of 1401 matches and he has been able to win 1149 out of them, which takes his overall win percentage to 82.
Nadal, on the other hand, has played a total of 1063 matches out of which he has won 877. The Spaniard's overall win percentage is 82.5, which is better than that of Federer.
The difference doesn't that much but we are talking about two players who are in a league of their own, so even a minor difference is significant.
Who Are Roger Federer's Kids? Know All About Federer's Twins