When playing a much, much higher rated opponent, it is critical to challenge him early in the match.  If you are a 1500-rated player competing against a 2000-rated player, you will likely lose.  If you play your normal game (statistically speaking), you don’t have a huge chance of winning.  This is the reason that you need to take some risk and CHALLENGE your opponent.
 
Assuming that you are a 1500 looper and your opponent is a 2000 looper, here are some possible ways of taking slightly more risk and challenging him…
 
#1 Normally, you might attack some long serves and push some long serves.  This time, you might need to force yourself initially to loop ALL long serves.
#2 Normally, you might loop very controlled on the first three loops before driving the ball harder.  This time, you might need to loop STRONGER on the initial ball to force your opponent into more passive defense.
#3 Normally, you might block when your opponent loops.  This time, you might need to block the first ball then go for a COUNTERLOOP on the next ball.
#4 Normally, you might play away from the table with controlled topspin.  This time, you might need to force yourself to STAY CLOSER and pressure your opponent.
 
When I say to challenge your opponent,
I don’t mean…  going for wild shots. 
I don’t mean…  trying to flip-kill every serve.
I don’t mean…  trying to backhand counterloop every shot.
I DO MEAN…  pushing yourself to take necessary risks to put added pressure on your opponent.  Once you win the first game and your opponent becomes nervous, then it is easier to continue playing well.  If your dominant opponent wins the first game and gets a 6-1 lead in the second, then he will likely mow through you to a 3-0 win.  Taking reasonable risk and challenging your opponent EARLY in the match is a critical aspect to pulling out that huge upset that you have always dreamed about.