Tennis, at the most elite level, is incredibly physical. The very best players may be some of the best athletes in the world ... in any sport. That's why when I hear the simplistic statement "at the top level of tennis, everyone has the physical skills to succeed. The difference is mental," I wonder what sport they are watching or if they have ever even watched any other sport.
Every top player may have the physical skills to succeed to some threshold, but everyone does not have the physicality to be the best. The limitation to success in any sport is not just mental. No one suggests that Lebron James simply has a mental advantage or what separates Usain Bolt from the rest of the other sprinters is the result his mental ability alone.
While the mental, emotional and intellectual aspects of tennis are very important, there are real physical differences in the size, speed, quickness, agility, power, coordination, fitness and conditioning of the the very elite. Rafa and Serena may both have the heart of a champion, but they are more physically capable too.
If other sports like football were all mental, then the NFL combine would be limited to an IQ test. If tennis were all mental, then I guess you would train in a classroom instead of a tennis court.