While I always urge players to compete to win events, let's face it - one of the great thrills of table tennis is pulling off a great upset. So let's look at how to maximize your chances of doing so.
First, let's define "great upset" as beating someone who really is much better than you. If you've improved a lot and beat someone rated a lot higher only because you are underrated, that's not really a "great upset"; that just means you've gotten better. And that, of course, is better than pulling off a one-time great upset!
But whatever your current level is, you still want to maximize your chances of pulling off an occasional big upset. How do you do this? There are three main reasons why big upsets take place. They are: 1) the weaker player plays great; 2) the stronger player plays poorly; and 3) the weaker player wins because of a style advantage.
Note that tactical play comes into play in all three. The weaker player may play "well" because of smart tactics, the stronger player may play "poorly" because of poor tactics, and the weaker player may have a style advantage only because he plays smart tactics to make use of that style advantage, or because the stronger player plays poor tactics.
The reality is that most major upsets involve at least a little of all three. You have control over only how well you play, and so a key to pulling off upsets is to simply play well, so that opportunistically you are ready to win if the stronger player opens the door by playing poorly, or if it turns out you have some sort of style advantage.
Let's look at all three of these aspects.
1) Weaker Player Playing Well
Some would argue that the key to beating stronger players is to play super-aggressive and hope you get hot. This rarely works, and usually just makes things easier for the stronger player. What does work is to simply do whatever you do best, but do it as well as you can - while at the same time having at least one shot that consistently wins points against the stronger player. You don't need to force the winning shot over and over; if you do, you'll just start missing. But it needs to be there when you need it, and you should maximize how often you can use it. It might not be a one-shot putaway; it might be a series of shots, such as an aggressive backhand, quick blocking, or steady looping.
Psychologically, you need to go into the match really believing you can win. You really should do this in all matches, no matter how good the opponent is. Even if you can't beat someone, you'll do a lot better if you play believing you can, and play accordingly. This puts you in a perfect mental condition to win - you won't start thinking about the big upset you are about to pull off (and thereby fall apart) because you'll be expecting to win.
You also need to get into "the zone," that mental place where you are playing almost unconsciously, where everything happens naturally. Most players have had this experience sometime; they key is to reproduce it while playing. Once in the "the zone," you can maximize your level and your chances of winning.
It doesn't matter whether you are about to pull off a big upset because the opponent is playing poorly, you are playing well, or if you have a style advantage; in all three cases if you start to think about the upset you are about to pull off, you will likely be pulled out of the "zone" you were in, and your level of play will drop.
2) Stronger Player Playing Poorly
You can maximize the chances of the stronger player not playing well by throwing variation at him. If you give him the same serves, receives, and rallying shots over and over, he'll get used to them, get into a rhythm, and mostly likely play his level. So perhaps throw more variations than normal against him, especially with your serves, and keep him off balance. It's a common way to blow a match, where a player finds something that works, but so overuses it that it stops working. You want to stick with what works, but you also want to stretch it out so it lasts the entire match. At the same time, once you find something that works, you want to maximize its usage without overusing it so that he gets used to it. It's a fine balance.
If you do find something that gives the stronger player trouble, here's a simple way to maximize its usage without wearing it out: use it early in a game, again in the middle, and then come back to it at the end. Often I've seen players hold back on a tricky serve, a heavy push, or something else until it's close, when if they used this earlier, the game might never be close.
3) Style Advantage
If you have a style advantage, you also maximize your chances of winning if you go in truly believing you can win. Psychologically, when you are about to beat a stronger player it's very easy to start thinking about it and thereby fall apart. Instead, if you really have a style that gives the stronger player problems, convince yourself of the truth: he is not a "stronger" player, except perhaps against other players - and he's not playing other players, he's playing you!
However, one of the things about stronger players is that they often adjust. This means you also need to make adjustments as the match goes on to keep that style advantage. If there's something you do that gives the stronger player problems, he'll likely look for ways to avoid that, even if it means playing his "B" game. Your job is to either find ways to keep using whatever gives him trouble, or to find ways to beat his "B" game. In the latter case, your style advantage has already done its job, forcing the stronger player to play this "B" game, and suddenly he might not necessarily be the "stronger player."
A key aspect of having a style advantage is knowing what gives the stronger player trouble, so you can use that part of your style that does give him trouble, i.e. the style advantage. Sometimes this will happen automatically, if your styles just happens to be one that gives him fits. More often it helps to do a little scouting, and find out what gives that player problems. Watch him play, ask around, experiment early in the match, and learn all you can. Information is power, and often leads to victory.
There are few more tactical matches then a clash that occurs when a stronger player faces a weaker player with a style advantage. Can he tactically adjust? Can the weaker player tactically keep using whatever it is in his style that works, or force the stronger player into his "B" game? Matches like this is where you need to put on your tactical cap and win the tactical battle. At the same time, remember that tactics should be simple. Don't overthink; find a few simple tactics that work, adjust them as needed as the match goes on, and focus on playing your best.