Does it surprise you that the best singles players are not usually the best at doubles? The term 'doubles specialist' is meaningful, and not derogatory, because there is a big difference between the two games. The most effective shots in singles are not necessarily the most useful on the doubles court. Why is that? What are the big differences between singles and doubles?
In doubles the amount of court needed to be covered by each player is less.
The court is wider. Wider angles can be hit than in singles.
It is very difficult to hit a groundstroke winner in doubles.
It is difficult to hit past their outstretched reach when two players are at the net.
The return of serve in doubles has to be hit into a smaller area to avoid being intercepted by a net player.
THE DIFFERENCE WHEN SERVING
In singles a well placed wide serve forces the receiver off the court, leaving the court wide open for the server. In doubles, a wide serve opens up the court for the receiver, and forces the server's partner to cover the down the line pass, leaving the middle more vulnerable.
A slower, more controlled first serve to the 'T' gives the server more time to serve and move in to volley, and the receiver fewer angles to work with. A fast first serve in singles often forces a weak return. In doubles a returned fast serve can put the server in a more difficult position, especially if it was a serve and volley attempt. The serve will have to volley from further back, a much more vulnerable position.
THE DIFFERENCE WHEN RECEIVING
In singles, an effective return of serve lands deep in the centre of the court preventing the server from dominating on the second shot. To get depth it should be hit with topspin with a good shape, high over the net.
In doubles, a similar return would be punished by the net player! To prevent the server's partner from intercepting the return, it needs to be hit into the small cross court window, low enough over the net to prevent the server from closing and hitting an aggressive volley! Low, chipped, short returns force a serve volleyer to hit a defensive volley.
A lob can be an effective return in doubles, to take away the net from the serving team. Be aware of how close the server's partner is to the net. Returning down the line makes for a very difficult overhead if it is reachable! In singles how often have you hit a lob as a return of serve?
THE DIFFERENCE WHEN HITTING GROUNDSTROKES
When hitting from the backcourt facing two net players it is important to keep the ball low over the net. Accuracy and placement are more important than sheer power. You have much more room to hit a passing shot against your singles opponent! Flatter hit balls are more useful in doubles. A down the middle shot often works when the net players both think the other player will attempt it.