You know what is the key to a good tennis shot? do you know what is the difference between a newbie tennis player and a tennis pro?
Run, get in balance, hit the shot, get back in balance.
Just like it is important to be in balance in life, balanced pose is the key in tennis as well.
The difference between a clean shot and a awkwardly hit shot.
Balance not only helps your shots but also helps one avoid injuries of all sorts
Now balance is not everyone's cup of tea, I am talking from experience, it is not even understood by beginners and even when after you start getting the concept it is not easy, because before hitting a shot, you need a little time to get in balance, you need to have that stamina, quickness.
and this comes only from enough practice, physical training like running or swimming and not the mention core strengthing excercises. Talking about core, a stronger core means a well balanced body. There are lots of exercises for a strong core plus yoga, tai chi naturally make you a well balanced person.
Go get in shape, be a well balanced person, play better tennis!