This is a triviality. Gravitational force is proportional to the inverse of the square of the distance from the earth's center (discarding relativistic effects). Hence on Madrilenian "clay" courts, each time the ball bounces on the ground, clay flies upward without hindrance and so slowly diminishes the number of clay particles on court. Just by itself, this would be a relatively minor problem that could be solved by watering down the courts at regular intervals. What forces the issue is the Spanish pants. If you haven't observed before, the pirate pants worn by players such as Nadal and Lopez are made of a special material that exerts an attractive force on clay particles. At sea level, this is really no problem; it only explains the Spaniards' natural pant-induced affection toward clay. But at high altitudes, the issue comes entirely into its own. The attractive force overcomes the weak gravitational force on the clay particles as well as the weight of any water particles! On players like Lopez, the clay particles forms a sort of shield around the body. On players like Nadal and Ferrer who are constantly moving at high speed, their body torques generate a centripetal force, ultimately leading to clay rings around their bodies, similar to those that form around the planet Saturn. Such body rings could cause Nadal and Ferrer to lose a couple more games than at sea level, hence the concern.