I agree that perfect technique goes a long way.
Most ATP tennis players don't try to develop that much of an upper body because it inhibits them to hit a loose stroke.
They still work on upper body like the shoulder,forearms,and wrists to keep themselves injury free.
Federer's upper body might not be as big as other pros on the tour but he has HUGE legs.
Physical wise most of your power comes from legs&core.
Remember you can be one of the strongest guys around but if you don't have good technique all that strength won't do anything for you.