In my consultations with tennis players I will often refer back to Cliff Lee, in his first stint pitching with the Philadelphia Phillies, talking about his Game 1 World Series performance against the New York Yankees. On the road Lee shut down the strong Yankee offense. When asked afterwards how he was so calm Lee talked about having put the work in practice to be prepared - and then just trusting that he was ready.
Dan Rouhier posted about Lee's confidence under pressure and that post Game 1 interview on the Ahead in the Count blog.
How Do You Handle Pressure?
I relate this story to tennis players because they often struggle under pressure with hitting the kind of ball they normally would. If you have put the work in you can trust that the game will be there when you need it. Your balls will land in the court. Just go for it!
As a coach remind your player that all he or she needs to do is trust their stuff and play their game. While this may seem simplistic it is the kind of reassurance and simple thinking needed to excel in pressure moments.