The potential for injury in tennis is great: the uncomfortable twisting and lunging as you reach for difficult shots; the vertical leaps for overheads, with the resultant pounding and jarring as you land; the unexpected sprints to the net and sudden stops as you go for drop shots; and the list is endless.
Thus, the probability is significantly high that at some point in your career as a tennis player, whether you are an amateur or a professional, you will get injured. It is therefore imperative that you strengthen your body. In addition, it is also a necessity to work on maintaining your conditioning and flexibility.
You achieve the flexibility by warming up, which in turn adequately prepares your body for the rigorous demands of the sport.
Many players at the amateur level do not appreciate the value of warming up. You do not have to spend an inordinate amount of time warming up, ideally your warm up should do two things: raise your heart rate and respiratory rate, without causing fatigue. You can get a good warm up in 5-10 minutes.
You want longevity and you gain that by protecting your joints, this is achieved by strength training and warming up.