Now that indoor tennis is almost upon us again, many singles players who have enjoyed the low-cost or free use of outdoor courts all summer will be joining doubles groups for the winter. At $16 an hour for prime time, playing singles indoors is like having cavier for breakfast.

Switching to doubles is both fun and tricky. For the first time in months, you can really angle your cross-courts without fear of hitting out of court. After all, you have nine more feet of playing space to work with.

On the other hand, there are not one but two people staring at you from the other side of the net. Your target area on any given shot is actually smaller than in singles -- except for pinpoint passing shot. Offsetting this is the fact that you have a partner to cover the return -- letting you hit for angles you might be unable to cover if playing alone.

One of the most overlooked keys to good doubles, especially when switching from singles, is the warm-up. The classic warm-up consists of using two balls and hitting only to the opponent directly opposite you. Many amateur teams put only one ball into play and hit it all over the place. By hitting down-the-line, using only half the court, you get used to doubles' more narrow target areas.

One great drawback, though, is that this overlooks the basic stroke of all doubles matches: the cross-court. While it's uncommon and unorthodox, we recommend that after ten minutes of on-the-line warm-ups, both teams agree to take ten minutes of cross-court warm-up. The chances of the two balls' colliding in midair are minuscule, and the extra running will remind you how wide the doubles court is and how deep the corners are.

Don't forget to practice volleys, the stroke that wins most points in doubles. If both membrs of a team come to the net together, there's no reason not to practice cross-court volleys as well as down-the line. t

If you've ever watched the pros warm up before doubles, you'll see that they take a lot of service practice. This, too, is especially important in doubles; even though the service court is the same as in singles, the presence of the second opponent puts pyschological pressure on the server and makes the little box seem smaller. Each player should hit at least six serves into the deuce court as well as the ad court. This service practice can go on simultaneously -- all four people can practice at once.

Q. the net man on our opposing team is a very good poacher -- cutting off my service returns at the net. How should I play him ?

A. Mix him up. Try lobbing over his head or occasionally service return down the line behind his back.

Our opponents have booming first serves. Where should my partner play when I am receiving serve ?

A. On first serve, have him play at the baseline. If the cannonballer misses, put pressure on his second serve by moving your partner up to the service line.