If we assume that beginner/lower-skilled players usually need to work on physical aspects ("fundamentals") of the game, and then progress to-and-through tactics/strategy for mid-tier play, what is it that you would say high-skilled, or even professionals need to improve? I would guess "mental toughness" is something, but what else? Once a very high level is reached, is simply maintaining that level the biggest thing--assuming diminished returns for improvement?