Great analogy to a pitcher.
Although most would agree pitching a baseball is tougher on the arm, tennis players too often serve day after day, only to wonder why their arm is starting to get sore.
Sometimes "less" really is more.
Tennis players as well as pitchers walk a fine line between practicing enough to be tops at their craft and crossing the line into an overuse injury.
Are you doing the Thrower's Ten exercises to lessen your chances of an arm injury? http://www.muhlenberg.edu/pdf/main/a...throwers10.pdf