"Concentration takes mental energy, as anyone who has fought off five break points before shanking a ball on the sixth knows. And whenever I saw Federer on the grounds, he seemed to be using as little of it as possible. Practicing with Nicolas Kiefer on Ashe a few days before the tournament, he mostly just messed around. He would hit a few familiar Federer shots, the heavy forehand, the penetrating slice, then shank a ball and grin, or yell. Either way, he wasn't really concentrating all that hard."
https://tennisworld.typepad.com/tennisworld/2007/09/the-mind-and-th.html