Tennis has dominant players in both the men's and women's games.
But one day they will retire, and the challenge is to predict the players who will take their places.
But there are a few retired players out there who are young and good enough to compete at a high level on tour.
They retired because they had had enough of the competition, no longer felt able to compete at the top level, suffered a career-ending injury or just got too "old".
Let's take a look at 10 retired stars who would still make the top 50, or better.