I agree he hits groundstrokes in a unique way, but this also wouldn't be possible without his fanatsic athletical talent. You have to have a lot of power in the upper body to execute certain shots like he does and I meant that this physical strength is his main talent, because he never really did that much to become this powerful.
Funny I saw these words by Jack Kramer in Tignor's post about Fred Perry :
Kramer claimed that Perry had, unwittingly, “Screwed up tennis in England...The way he could hit a forehand, snap if off like a ping-pong shot—Perry was a physical freak. The kids there copied his style, and it ruined them.”