Windjammers is no sport simulation: on clay, your movement won't be impeded like they are when playing tennis on that type of surface.
Clay is a large arena, with a standard score layout (5 points in the middle, 3 points on the sides) but with 2 bumpers close to the center of the court.
This configuration makes it the most "love or hate" stage of the game: some people consider it's too random but yet might still pick it cause well, "with a bit of luck I could win". But most would run out of luck.
The more you play on Clay, the more you discover it allows for new ways of playing, of doing things differently whether it's on defense or attack. You're never gonna get bored.
These bumpers also cause Biaggi and Mita's super costums as well as many super spins to go crazy. But it's not random: everything can be reproduced by chosing the right effect in the right location.