Sharapova's shots are really fast, but... a fast ball means the ball also comes back at you just as fast.
Literal
Sharapova [gen-の] shot [topic-は] very fast [copula-のですが]... fast ball [quot-というのは] that-amount fast ball [subj-が] return-[come-くる] [explanation-んです].
その分 'that amount / correspondingly / to the same extent' is a stock adverbial for proportional reasoning — 'X correspondingly produces Y.' 返ってくる 'to come back' uses ~てくる in the motion-toward-speaker sense. というのは sets up the definitional/explanatory 'what a fast ball means is...'. んです gives the explanation a conclusive polite tone.