You're winning. That's good. You're composing your victory speech. That's bad. You can still lose! Think about how devastating it will be to have been winning & then it all disappears. It can happen. It will happen. Unless you are so scared that you still might lose. The fear will keep you focused. Concentrate on your breathing. Forget the score. Play so hard on every point. Go for everything. Take nothing for granted. You still can lose no matter how far ahead you are. It ain't over til it's over. Fear of losing is a great motivator.
For more tennis tips go to Joel's Tips Page