It is hard to say whether Federer is the GOAT because the game of tennis has changed a lot since the era from about McEnroe to Agassi. Changes in equipment and the ball have narrowed the differences between clay, hard court and grass. Also, the game has become almost exclusively a baseline game. Serve and volley is dead. So, the game is much more about endurance and consistency than it is about having a huge serve and soft hands at the net. Federer is definitely the king of modern tennis. Nadal might have overtaken Federer, but Nadal's style early in his career was too dependent on pure strength and his superior agility. Nadal had more power and could cover more court than Federer, but Roger had much better endurance because he was so incredibly accurate and consistent with his stroke that he never had to run as hard as Nadal or hit as hard because Federer is always relaxed and totally in control. This is why Federer has never had to retire in a match and has had very few injuries over a very long career. Nadal blew out his knees badly and beats himself up pretty badly on the hard court. He was playing great tennis in Oz, but the pounding on the hard court caught up with him.