Actual motor skills take a while to fade. Conditioning goes first, then reflexes. If you've trained long enough so that you can throw a good clean technical punch or kick, then that skill will last quite a long time even if you don't keep up your practice. Your timing will lose its precision, your reflexes will get slow, and you'll be sucking wind the first time you come back to a hard training session, but the fundamental movement skills don't disappear that quickly.
As far as a professional athlete taking time off and coming back "at the top of his game", you have to remember that elite professional athletes are typically pushing the limits of human capacity and their success or failure is determined by tiny percentages in their performance. An athlete operating at 50% of potential is a hobbyist. An athlete operating at 80% of potential is a high-level amateur. An athlete operating at 95% of potential may be a professional. An athlete operating at 99% of potential is an elite professional. An athlete operating at 99.9% of potential is likely a champion. Maintaining that last few percentage points or fractions of a percentage point does require constant practice and losing a couple of percentage points can mean the difference between a loss and a win even if the athlete retains 95+% of his skill.
