Usually any profesionnal player has superstitions or obsessive-compulsive disorder. Some players believe that a special movements, actions or objects will help them win. Tennis is no exception. Many players have "things" that they do before, during or after a game.
Andre Agassi forgot his underwear once. He won the game going commando. He never wore underwear again during his games. Rafael Nadal is probably the tennis player with the most OCD's. Here is his serving ritual: He will start by clipping the floor behind him with the tip of his foot.
Then he picks his shorts, tucks his hair behind the left ear, then the right ear.
He wipes his forehead, bounces the ball one last time and only then will he serve. And that's only one out of many.
Serena Williams will always make sure the members of her players box are in the exact same place every match. Also once she wins a match, she will keep the same socks until the end of the tournament.
Justine Henin, Ana Ivanovic and Maria Sharapova will never walk on the court lines.
Roger Federer has an obsession with the number 8. Everything has to be in eight. 8 tennis rackets, 8 bottles of water, etc.