I will tell
you specific training for tennis,
specifically for tennis.
Ironically it comes from other sports.
baseball - one of the best tennis players I know used to play baseball. A racket is just an extension of the hand. He can handle the ball in innumerable ways.
volleyball - the best server was a volleyball player, he's 1 of 2 players who can put the ball through the fence, the other is a martial artist
ballet - for the flexibility you need when you slip on a grass surface and you won't be getting up by the count of 10, ala boxing.
boxing - a balanced structure all the time, and delivering the uppercuts and roundhouse with balance and your whole body behind it.
soccer - endurance, footwork, I know a soccer player who can regularly hit a tennis ball behind his back, between his legs.
track and field - obvious, speed and endurance
All these are sophisticated skills. And
none of them you can learn from anything like
www.tennisfitness.com.
Speaking of baseball, from what very little professional tennis I see, I see tennis players are serving like a baseball pitcher with the cantilever leg (that's also how you throw a person in judo too.)