From the Super Mario Wiki
Mario pulling out his Iron Hammer
Iron Hammer is Mario's Offensive Power Shot in the game Mario Power Tennis. Mario summons a huge Hammer and hits the ball with tremendous force with flames trailing behind it. This will knock the opponent to the far back of the court and leaves them open to a drop shot. There is one weakness to this move however. If Mario and the opponent are standing close to each other, the opponent can quickly hit the ball over or to the side of Mario. It can also have a good effect, hitting the opponent so far that they can't react, thus giving Mario or his team a point or hitting the opponent directly thus giving Mario or his a point. It has good power but slow recovery time. Being hit with this Power Shot will also cause the opponent to lose a Power Shot he or she may have stored. Also, if Mario is right-handed, he will jump and hit the ball from the left side, but if he's left-handed, he will jump and hit the ball from the right side.