You want the ball to appear in focus when you hit the ball which you cannot do if you are far sighted and wearing glasses. But if you don't wear glasses or contacts (which I don't while playing) you don't see the ball well coming off the raquet of your opponent and it costs you in reaction time. This is why most tennis players and baseball players are done at 35 imo. Their eyes are starting to change almost imperceptably and they cannot track the ball like they once did.