The coach (or a partner) is inside the ad service box – about half way between service line and net, and with the inside foot stepping on the center line (see graphic below); a basket of balls is nearby. The player (A) begins the drill on the opposite side, baseline center.

Drill:
The coach will feed a wide ball as close as possible to the player’s right singles side line; the pace of the ball should be adjusted according to the speed capability of the player. As soon as the ball is fed, the player will sprint to the ball and considering hitting the ball on the run, from a position somehow outside the singles side line, he’ll have to choose the following defensive targets:
– sharp cross-court;
– deep down-the-line;
– deep cross-court lob;
– lob down-the-line.
In evaluating his options, the player must consider also the height of the ball before the contact and how early he can get to it which will determine his level of balance in executing the shot.

Note: This is a good drill that eventually teaches the player not only to think and react quickly in making the proper decision but ultimately he’ll have a clear idea of what his options are when his opponent comes up to the net behind a wide ball.
Do the drill on the backhand side too.