What if a player devoted himself to practising only and exclusively his serve 12 hours a day since an early age? He would become terribly proficient at it and could hit balls with oustanding power and fail-safe placement forever. He would be unbreakable. Then, as he would suck at pretty much the rest of the necessary skills to play tennis, he would have to wait for the tie-break and hope for an unforced error from his rival on serve. I know it sounds crazy, but we all know how good one-dimensional people are at their thing. For example, the random guy who can score thousands of free throws in a row, or the one who can juggle with a soccer ball better than any top player.