All sports have 4 main areas - technical, tactical, physical and mental. Tennis is no exception and the mental part of tennis is usually the deciding factor that tips the scales in the match. Fed is perfect in the 3 other areas ( that's why he is the BEST/on his way to GOAT status) but I think not winning the French Open is more of a mental thing for Federer. Don't get me wrong, Fed is one of the mentally toughest players I have ever seen but it seems he plays tentative, less agressive and error prone tennis on clay and still does not have that belief deep inside that he can win at Roland Garros. Federer is the ultimate tennis player ( he has the most complete game among past and present tennis champions ( men and women) and has the game to dominate and beat Nadal at Roland Garros. But as of now, the mental aspect is preventing him from winning the French.