This saying has been repeated so much it may hurt, but read it again. Tomorrow's a mystery.
Yesterday is history.
It's certainly true in playing a tennis match. We coach players not to worry whether they will win or lose a match, or what happened in the last point. Rather, they need to be in the present, or process oriented rather than result oriented. For the tennis teacher it is also an important reminder. Teachers should ideally be in the present with their students, attentive to their student's needs and not mentally wandering off onto the next court or thinking about their schedule later in the day.