During warm-up look for your opponent s weaknesses and strengths. When the match starts, put a lot of pressure on the weaknesses. If your opponent comes out red hot, and you're down in the match, find a way to make it last as long as possible. Often, your opponent will cool down and you'll get better. Make the match last as long as you can, because you still have a chance of winning.
Tennis is a psychological game.
You always have a chance to win. Play your game, and exploit your opponent's weaknesses. Get into the game. If your mind is not focused, your body won't be either. Concentrate, focus and show some life on the court.