When you talk about 'turning back the clock' the number 1 issue at hand, and the most important is heart. Without heart, a professional athlete, regardless of the sport, cannot make a comeback. From the 3 athletes you posted above, Safin is automatically out (I can't believe I'm saying this), everybody knows Safin is more interested in the girls than tennis. Roddick and Hewitt are both hungry, but i think Roddick wants it just a little more...he's shown he is capable of working hard (He's lost ALOT of weight since back then, and has worked with multiple coaches to incorporate a more complete game, rather than just a serve). Hewitt, although he has heart, his game is just too passive in this day and age. In order for Hewitt to make a comeback, not only must he retain his love for the game (not to mention that Hewitt is the only one out of the three that has a family), but he must also change his play style, which is almost impossible at the stage he is in his career.