There's no question that GSP's by far the better fighter in pretty much every aspect off MMA but Serra threw that punch, landed it and finished GSP and IMO you can't call that luck.
Coleman beating Shogun was luck, Serra beating GSP just isn't as far as I'm concerned. Hell I don't even like Serra but I think it's unfair on him that people call him lucky for winning that fight. He knew he was outmatched so came out swinging, he might have caught GSP on an off night but that's a part of the game. He planned on throwing some bombs and catching GSP early and thats exactly what he did, and finished him.
Haha, it seems to me that you practically defined a lucky punch.
If I were to get a chance to fight say Chuck Liddell. I know that he would murder me. Therefore, my gameplan is to come out swinging, hoping to catch him with a perfect punch to drop him. By chance, I do it. According to your definition, that would not be a lucky punch, just... me being on, while chuck is having an off day????
I call a win lucky when the fighter that won is completely outmatched.
Let's put it this way. If Serra and GSP were to fight 10 times, how many do you think Serra would win? For me, it has to be 1 out of 10 (and the only reason I give him 1 is because he friggin did it). The odds aren't with him, therefore he was lucky.
lucked, luck·ing, lucks Informal
To gain success or something desirable by chance: lucked into a good apartment; lucked out in finding that rare book
Basically, he didn't beat GSP because he is a superior fighter, he beat him by chance that a hard shot, would land at the right time.
Now, this doesn't apply to guys that are KO artists like Chucky because they train their striking through and through and have shown, through positive outcomes, that them getting a KO is not simply luck or chance. It is likely based upon their skills.
You can define it how you like, but when an inferior fighter beats a superior fighter with KO early on, it is luck in my book. They didn't dominate their opponent, they hit him with a great shot through luck/chance.