In tennis, strength is used to generate speed, power and endurance.  It is impossible to have agility, speed, power, a developed anaerobic system, and flexibility / mobility without optimal strength levels. Strength training is also critical to prevent injury that can occur from the repetitive movements that can lead to the development of muscular imbalances which significantly increase the likelihood of injury.  Thus, specific strength training for tennis is essential to maintain or restore proper muscle balance.