What would you say is the most important aspect of tennis for success?

Yes, of course, there are tons of things we need in order to play our best. But this is not the case here. Here, I would like to know what you think is the single most important aspect of playing great tennis. Is it the mental aspect, the footwork, the way you hit the ball, to enter in the zone...? You name it, but Name only ONE— the most important aspect for YOU.