Tennis is a moving sport; that should be obvious to anyone who's ever picked up a racket. However, often times players do not make the best use of their practices in order to achieve both technical expertise as well as physical conditions. Dollars to doughnuts, most developing players will spend 2 hours practicing tennis and then performing a fitness routine (maybe cardio, weights, jumping rope, stretching, etc.) for 30-45minutes thereafter.
What if there's a better way?! There is. For maximum benefits, try performing your fitness routine DURING practices. For example, players could jump rope while the coach is collecting the tennis balls, sprints could be run in between drills, push ups, kangaroo jumps, etc could be immediately performed for missed shots. Performing fitness exercises during tennis practice will ensure that the tennis component is improved at the same time as the fitness component. In other words, players need to understand that fitness is not secondary; it is as important as the rest of the game. However, perfuming exercises at the end sends the message that they are less important (same message is sent when serves are practiced during the last 10 minutes of practice). Furthermore, if the player performs fitness exercises during practice - particularly points - his level of performance will skyrocket. It's one thing to play points or do drills when you're relatively fresh; another to do the same while you're fatigued and sucking oxygen.
A great way to improve your tennis fitness would be to play a set where the players have to run a sprint (1minute; e.g., ten-ball sprint) between EVERY single game. This will ensure that the player's conditioning will not break down throughout the match. Of even greater importance would be to run 2 sprints before the ever-important 7th game. If you can hold serve while fatigued, well then you're a player. Similar concepts are implemented by the elite military forces throughout the world. Some units perform tough physical exercises and then are asked to perform some mental task in order to determine how the soldier thinks under the pressure. Elite tennis players would benefit from the same type of training.