Fitness/focus issues are very, very important on this level of tennis. In their eras, Bjorn Borg and Ivan Lendl dominated as they were much fitter men than their opponents. The modern tennis player needs to train and have a very high level of fitness, and that coupled with talent decides how consistent a professional he or she will be. We see these issues with Eugenie Bouchard. I don't think she was a champion level talent but certainly a top 15 player in talent. Ditto with Anna Kournikova, she was actually top 10 in talent in her prime but the off court distractions to tennis derailed her as a professional. I never thought Jim Courier had much talent but he became number 1 in the world due to hard work and fitness.
I think the younger players on the men's tour struggle with distractions and there has been a large number of flashes in the pan. Marat Safin was a guy I thought could be great, but he burned out. Mark Phillipoussis was another one. There have been many like these two.....on the other side of it you see a hard working journeyman like Wawrinka who played his best tennis in his late 20s and early 30s and even won a couple of majors. To me Wawrinka is the classic late bloomer. Arthur Ashe was similar......played his best tennis late in his career before developing heart issues and crushed Jimmy Connors at Wimbledon in a massive upset in 1975.
