I was wondering what are some good ways to workout in the gym to be better fit for tennis. Mainly I want to get more waist and shoulder rotation as well as developing a faster wrist action on serves.
Thanks for the input ^.^
"Waist rotation" as you call it, is rotation of the core around the legs.
By far, the single best gym exercise to strengthen the core muscles, [and so very importantly, their connection to the legs] is the squat. Doing squats with increasing weights will actually increase the strength of these muscles.
http://www.youtube.com/watch?v=x6RIs8-AVwM&feature=related
But mere increases in strength won't immediately translate into harder groundstrokes and serves. That will come with practice to develop muscle memory for the stronger muscles to fire quicker doing coordinated movements. So on your groundstrokes, out of open or semi-open stances be sure to fully coil and quickly uncoil to power your shots. On the serve, be sure to both coil and develop a full "bow" position as you go into your trophy position, and then uncoil and unbow as rapidly as possible after your big leg push off.
Many find that throwing a medicine ball sideways helps mimic the explosive leg/core/arm sequence that are involved in groundstrokes.
http://www.essentialtennis.com/video/2011/05/tennis-fitness-medicine-ball-workout/
If you look at the Health and Fitness section of Talk Tennis you will see way to many who have suffered from torn rotator cuffs and labrums. Don't be one of these. Do the thrower's ten exercises
http://www.eteamz.com/palyvolleyball/files/throwerstenexercises.pdf
There have been many threads on the wrist action on serves, and most will agree that almost all the wrist action is a "passive" ulnar deviation that results from holding the racquet with radial deviation until the momentum of swinging the racquet up releases the wrist in the powerful "pronation" movement ["pronation" being the tennis term of swinging up at the ball with the side of the racquet and turning the head of the racquet into the ball at the last second with an external motion at the shoulder]. Work on this "pronation" move at the tennis courts with this exercise:
http://www.youtube.com/watch?v=iONY6fcqZGg
Since the serve is the culmination of involving the muscles of the legs, core and arm in a smoothly coordinated kinetic chain, do a full body strengthening program such as the following:
http://www.sport-fitness-advisor.com/tennis-weight-training.html
Then practice, practice, practice to coordinate that strength into a serve where the whole is greater than the sum of its parts.
And certainly don't forget that it is the fast, wiry guys like Djoker and Rafa that usually beat the stronger but slower guys like Soderling and Isner. So make plenty of room in your training for sprints, HIIT and agility drills.
High Intensity Interval Training:
http://www.intervaltraining.net/hiit.html
USTA agility drills:
http://assets.usta.com/assets/1/USTA_Import/USTA/dps/doc_437_269.pdf
Good luck!