Cover the line when at the net

When you approach the net in singles, you usually want to cover the down the line shots first and be ready to cover the crosscourt. In this picture, it's easy to see that Roddick isn't very natural at covering the net and Murray has a wide opening for the down the line passing shot.

Roddick should be at least a full step closer to the line. His right foot should be where his left foot is. This would narrow the angle Murray has for his passing shot, but still put Roddick in a position to cover the cross court. Since down the line passing shots have a shorter distance to travel, they get by you faster, and you don't have as much time to cut them off. Crosscourts, on the other hand, have a greater distance to travel allowing you a fraction more time, and you can cut them off by taking a diagonal path toward the net.

In any case, it's pretty clear in this picture that Roddick is over-compensating for the crosscourt, and is basically giving up the down the line pass, making it an easy pass for someone like Andy Murray.