A lot of sports people do have these superstitions. I think it's all about establishing confidence and a feeling that you are in control. Imagine entering a high pressure contest against a player or team of similar ability to yourself. You are a great player, but so is your opponent. You can influence the outcome of the match, but you can't control it by yourself, because your opponent has a say as well. Supersititions of the 'if I do x then y' variety can help to establish confidence by creating a mental environment in which you have control.