From the Super Mario Wiki
Mario pulling out his Iron Hammer
Iron Hammer is Mario's Offensive Power Shot in 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 it open to a drop shot. If Mario returns an Luigi's Offensive Power Shot with Iron Hammer, Mario will not suffer from any knockback. Being hit with this Power Shot will also cause the opponent to lose a Power Shot he or she may have stored.