It's no wonder that hard courts have almost always been the most competitive field since inception. Even in this era, while both clay and grass still remain somewhat weak fields, hard surface on the other hand, remains the most competitive. We've seen both Nadal and Federer dominating clay and grass for a number of years now; whereas, all the wins by other players quite predictably have come on hard surface - be it Del Potro in 2009 USO or Nole at the AO multiple times or Andy at the USO and of late Wawrinka at the AO. Also the likes of Safin, Roddick, Hewitt had seen their success on hard courts first. It clearly shows, if anything, that hard surface is the most neutral ground for all the players.