Many tennis players have a misconception of achieving topspin on groundstrokes. I will try to clear the air in simple terms.
Topspin or overspin is when the ball is spinning over itself in the direction moving forward. Topspin can be achieved simply by contacting the ball with the racquet strings vertical to the ground and the racquet head moving up. Let me give some facts about topspin and then explain why.
-Topspin decreases ball speed. The more spin a ball has the slower it will travel. A tennis ball hit flat (no rotation) will travel faster than one with spin. Thus topspin allows for players to swing faster or hit harder and still maintain control of the shots placement.
-Topspin makes a ball drop quicker than without topspin. Overspin on a tennis ball creates pressure on the ball that forces the ball to fall to the court more sharply than without. This allows players to hit higher over the net (less net errors) and still bring the ball down into the court.
-The more forward the swing the less topspin. Likewise, the more upward the swing the more topspin. The key is to get the right blend of forward swing and upward swing into the shot. The forward swing of any stroke in tennis should come from body rotation. As the body rotates forward the arm and hand swing upward to produce the topspin.
-Tennis racquet strings must remain vertical during contact. Many people have the idea that you can roll the racquet face over the top of the ball to achieve the overspin. When a tennis ball stays on your strings for 4/1000 of a second it is impossible to do this.Doug Hofer, USPTA & USRSA