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.
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.
I wouldn't call it luck per se. GSP was simply ill-prepared for the fight and got TKO'd because of it. He was not mentally there and that's why he lost.
I guess Serra was lucky in the sense that he got the perfect scenario to fight GSP. It was his first title defence, he had a lot of hype surrounding him at the time, he was a huge favorite to win = tremendous pressure.