Suppose you are way behind in a game. It's likely that the only way of winning is if you play well and your opponent doesn't. Therefore, assume this is true, and play your tactics accordingly. The same idea can be used when playing a much stronger player - if you can't beat him unless you play well and he doesn't, then assume both, and play your tactics accordingly.
In both cases, assume your opponent is going to be inconsistent if he is regularly forced to go for difficult shots, and so you give him those types of shots. At the same time, you assume you will make your normal shots, perhaps with more consistency than usual. Don't overplay - that's the quickest way to blow a lot of points. Find the right balance where you put continuous pressure on the opponent while staying consistent.
Confidence is key to both coming back and beating a strong player. The moment you doubt yourself, you'll be hesitant, lose consistency, and won't play well. So just convince yourself you can play well at these times and in these matches, play your game, and let yourself go! (Hint - try doing that in ALL matches.)