A Millisecond Sport

When hitting the ground stroke, it is commonly believed that using more wrist while coming over the top of the ball will produce more topspin. But in observing Rafael Nadal or Maria Sharapova during the 2012 French Open it is clear that these world class players do not employ more wrist nor do they excessively come over the top.  Watching their strokes in slow motion we see that the wrist is laid back right before contact.

The top players have a long 'hitting zone" which allows good, solid ball contact.
The ball itself makes contact with the strings for roughly one-fourth of a second. Approximately the length of an eye blink.

It is important for recreational players to practice drop hitting the ball and lengthening the hitting zone. Visualize the ball traveling through a tunnel a foot in length. Let the racket stay vertical longer. 

It is quite possible to learn watching the pros but impossible to see ball contact. I recommend recording your stroke with a top grade digital camera set to slow speed. This will permit the eye to see that invisible millisecond of contact.