I have had assorted injuries via tennis most of my life. I had done weightlifting for years, but a recent article I read in Men's Health stated that men are better off to just do bodyweight exercises such as pushups, pullups, dips, squats, forward and reverse lunges, etc. The theory behind this is that when you use weights or machines, you are not doing natural body movements you do during a typical day, and predisposing yourself to injury. I was surprised to read that the special forces do not use weights or machines for this reason (this is what MH said anyway). That being said, if you have a weakness in one part of your body due to overuse, such as a weak rotator cuff from serving, specific weight training can help you rehab.