If, say, for the sake of argument, you were the third best time trialist in the history of cycling and your career spanned the 2000s, and the best two cycled in the 1990s, you would have been world number one for a decade. If, however, you just happened to have been born 10 years earlier, you would have never got to higher than world number 3. Put it another way. When Murray, Federer, Nadal and Djokovic have all retired, in say 3 years time, someone who isn't fit to lace their boots will be world number one. We live in a freaky era of tennis, where most of the world's best ever are all playing at the same time (men's game).