How many times have you lost a match and complained about how "weird" your opponent played? It could be a different surface that you weren't used to; a strange serve you weren't familiar with; or a weird stroke that gave you fits. If it's a "weird" surface, then adjust to it and learn to play it - with different colors, you always know what side he's using, so there's no excuse to get fooled.
In each case, instead of complaining about what the opponent did, you should think about how and why he did those things, and why you weren't able to adjust. And then, give the opponent credit for finding a way to win.
The whole point of playing to win is to find ways to win within the rules. If an opponent has found a way to mess you up, then he's done exactly that. He's earned his win, and by not adjusting to whatever he did, you've earned the loss.
Think about it. If whatever the opponent is doing is rare, then it's likely an inherently weaker style, or others would be doing it. And so, if you were unable to find the inherent weaknesses in that style and win against it, then you have literally lost to a player who handicapped himself by playing an inherently weaker style!!! Take advantage of his weaker style. If you can't, then you need to look at your own game and figure out why, with such an advantage, you were unable to win. It might be a problem with your technique; with your tactics; or your mental attitude. (Saying you weren't used to it only works for one game; after that, if you haven't adjusted, then it's on you.)
So, let's take this pledge: "I will never again complain about an opponent's style." Wasn't that easy? From now on, instead of complaining about losing to a weird style, focus on how to beat that inherently weaker style.