In singles, at all levels of play, the preferred direction for groundstroke exchanges is crosscourt, not down-the-line. Here are the reasons:

The net is six inches lower at the center. Down-the-line groundstrokes must pass over the highest part of the net.

The court is 4 1/2-feet longer diagonally. While the distance down the sidelines is a short 78 feet, the distance crosscourt is 82 1/2 (the width of the doubles alley.)

When you send the ball straight down-the-line, you must be accurate and "thread the needle." Crosscourt groundstrokes can be sent into the corner of a box.

Crosscourts minimize your court coverage. Your short, slow down-the-line shots tempt your opponent to angle the ball sharply away from you, possibly running you wide of your sideline. Your crosscourts, on the other hand, require your opponent to return back toward you or at worst, along your sideline. This saves you a lot of needless, hard running. Don’t open a can of worms if you don’t have to.

In singles, crosscourt is the way to go most of the time, especially when you’re defending. It’s a safe, percentage shot that keeps you in the point longer, minimizes risk and helps you conserve energy.

You can attack down-the-line; that’s another lesson.