I love this video from the Association of Tennis Professionals on how Andre Agassi was able to read Boris Becker’s serve through one of Becker’s small ticks: before he tossed the ball, he’d stick out his tongue; if he poked it out straight, the ball would be served down the middle, if he put it out to the left, the ball would go wide.
The part I found most interesting though was how Agassi used this information to his advantage. He says, “the hardest part wasn’t returning his serve, but rather not letting him know that I knew this.”
Agassi would save reading Becker’s serve until he needed to win an important point in a match, so he didn’t give away that he knew.
Agassi’s genius wasn’t in the “mind reading”, nor was it in returning the serve (although that was pretty hard to do too!), it was in knowing when to selectively, deliberately, and specifically use the information to his advantage.