What's more, a machine-made surface produces the wrong kind of friction: to do its job precisely, the plate's surface needs minute scorings, of only a few microns each, where machine oil can reside.

CNN: ASIANOW - TIME Asia | GizmoLand! | Man Beats Machine

But on a high friction surface like clay, the ball will grip more and change the trajectory of the ball quite significantly.

BBC: SPORT | Tennis | Rules and Equipment | On the rebound

The crushed-brick mixture that constitutes the base of a clay court produces a the highest bounce and most friction of the three surfaces, meaning they have the lowest CPR of all three surface types and are considered as the "slowest".

CNN: Tennis interactive: Why clay is so different