To be honest I can’t be sure because I’ve never met him but I have an idea why. Old English was a Germanic language that was influenced by the Old Norse language of the Vikings and then by the Norman French that came with William the Conqueror’s invasion of 1066. It was also influenced by the Latin of the Catholic Church. Mix it all together and eventually you got Middle English. A couple of hundred years later we reach Shakespeare and the beginnings of Modern English.
Roger Federer was brought up near Basel, in Switzerland and speaks Swiss German, German and French. This combination is perfect for learning English! Oh I forgot to say his mother grew up in South Africa and spoke Afrikaans. With this background Federer had a major headstart on most people!