Endurance is your ability to perform physical activity over time. You might be able to serve aces and hit winning forehands and backhands, but if your muscles cramp or you run out of energy after a few games, you won’t win any matches. You build endurance by exercising for longer periods without stopping, working at a slower speed than when you interval train. This type of exercise is usually aerobic, calling on your slow-twitch muscle fibers and burning more fat than glycogen, depending on the speed you work. The slower you work, the higher percentage of your calories burned comes from fat.
Many of the activities you do to build strength will also help your balance. Balance is partly a matter of developing your small stabilizer muscles that provide support and keep you steady. Free weight exercises, such as lunges and deadlifts, will help strengthen the stabilizer muscles. Even better, include some single-leg exercises, such as single-leg deadlifts and pistol squats, in your strength-training routine.