Cardio exercise for tennis players, off season and between matches

Improving your physical fitness side is straightforward; you need to improve your endurance by ideally a minimum of 30 minutes cardiovascular activity at the right level 3 times a week.
Avoid doing your cardiovascular workout before you play your game. Both weight training exercises and explosive leg exercises such as short sprints and plyometrics can improve explosive power. Remember to work within your own limits.
Aim to spend 1 day a week doing either sprint / plyometric work. As these are both very demanding, avoid if you have a low fitness level, or any injury.
For your sprint work, try and do the sprints on the court you play on, wearing the same training shoes you play in. There are many variations, however keep them short as for your sport.
Shuttle sprints, from baseline to serving box and back, then up to the net and back. You can repeat this with either no rest or short rest. For best results run against a partner.
As above but sprint forward, and run backwards coming back.
Ball retrieval, place a number of balls across the court, sprint to each one pick it up, return it to the baseline, continue until all balls are recovered. Try timing yourself.
Partner Ball Throw, Stand with your back towards the net or wall, with your eyes shut, have your partner throw a ball in the air behind you somewhere. When they say "go", aim to reach the ball as quickly as possible, you can also aim to hit the ball over the net etc. with your racket.
North South East West, Sprint North towards the net / wall, as you get half way, side step East to the sideline / wall, then continue to sprint North. On reaching the net / wall, run backwards to halfway mark, and side step West to side line, then run back again to baseline. Take care when running backwards for ostacles and to keep your balance. Start slowly and build up. This and other drills and techniques will get you into the best possible shape.