*assuming you're playing right-handed*:Originally Posted by WuZeMu
If you swing your remote early as the ball nears your player, you'll hit the ball left. If you hit it late, as in when it is almost level with your body, you'll either hit it straight or a bit to the right.
Another tip for tennis is to play a range of shots to get the opposition working. Don't just smack the ball all the time, try mixing it up. Start off with a few simple shots back and forth, then mix it up with some looping balls, some spin shots and some volleys. Get the other team working hard enough and eventually you'll be able to play a shot that they can't reach, or if they do they'll hit it out of the court.