Maria, would like to compliment you on your incredible grasp of the English language (has anyone mentioned it?). Everytime I hear you speak I simply cannot believe you only started to learn English at, what, 7 years of age? I've been formally taught 4 languages and lived in Europe and the Middle East for more than 13 years yet cannot even now mumble 10 words in any of them, so you can understand my awe of your language skills. I like to watch you compete (with the sound off); you appear to be a very bright and very classy woman and I wish you continued fantastic success.
The earlier you start speaking a language the better you speak it. The persons who speak very well a language are those who start speaking it during their childwood.
The fact that Maria Sharapova still speaks russian very well, it is due to the fact that she starts speaking this language very early.
When you live in a country where everybody speaks english, it is normal that you speak english very well particularly if you start speaking it at a young age.
The fact that Maria Sharapova has often to speak publicly when she wins a tournament also helped her to speak english better.
Anyway Maria Sharapova is not an english teacher but a perfect russian tennis player.