Better Shape = Better Tennis

In an ideal world, every player would focus on their body's fitness first, especially if they're going to be playing singles frequently.

First there is your aerobic fitness.  Your body needs to be able to last for 2-3 hours if you're an adult.   If you are a junior player, your body needs to be able to last 2-3 hours, fully recover within 1.5-2 hours and be able to again last 2-3 hours for a second match.    Consult a physical trainer regarding what specifically you would need to do to achieve the physical requirements from a cardio perspective.  It will likely entail running, an elliptical machine, and/or biking.  How often and for what duration will depend on your baseline physical condition.

Second is your anaerobic fitness.  Your body must be able to recover from point to point without being exhausted.  Tennis is a series of short bursts.  You must be able to fully recover within 25 seconds point to point.  Much of the work necessary to improve your anaerobic fitness can be done on court with your tennis professional.  By increasing over time the number balls you hit before a break, limiting that break to 25 seconds, and taking 90 second breaks after every segment, you will see your ability to recover point to point improve, even when points become very physical.  Anaerobic fitness can be measured - the measurement is called VO2 max.   

Coinciding with your work on your aerobic and anaerobic fitness should be some sort of strength training.  The frequency, duration, and intensity will depend on your baseline physical fitness, your age, and your goals.  A good generic plan is to first train your muscles for endurance/power by doing low weights (60% of your max strength) and high reps (20) for a set period of time.  Follow that by training your strength for a few weeks with reduced reps (8-12) at higher weights (80% of max).  The strength training will help your muscles withstand the physical punishment of a tennis match, by maintaining their strength throughout the match.  It is comforting to know your hard work will allow your serve to keep the necessary MPHs for domination.  Consult a physical trainer to set up an individualized program suitable for your needs and goals.

Your physical fitness directly impacts the quality of your movement, which directly impacts your ability to execute shots over the course of a match.  The less fit you are, the earlier your game will deteriorate during the course of a match.  Don't get trapped into thinking better technique will improve your game if you're getting tired after set one.  If your opponents can outlast you, they just need to survive long enough for your body to break down.  Reverse your thinking.  First work on your physical fitness, then work on the technique and/or tactics. 

Once you tackle your physical fitness, you can begin to tackle problems in the rest of your game.