Luck or chance: is fortune (whether bad or good) which occurs beyond one's control, without regard to one's will, intention, or desired result.
I would say Serra had control of his punches. And it wasn't beyond his control.
The thing is, people are not fully in control in a striking exchange, because the movement partly so fast that it is beyond the reaction time of their nervous system.
That's why you occasionally see great strikers getting KOed by mediocre strikers, but you will almost never see a great grappler getting submitted by a mediocre grappler (gotten beat up before aside).
Of course, it's not pure luck if a mediocre striker KOes a great striker, because he has trained to strike, but there certainly is a luck factor in it.
Striking is maybe comparable to poker, where you can train your calculating abilities, psychology etc., but still there is a luck factor. A good poker player would win most of the times, but even a complete beginner could win if he gets good cards. Grappling is more like chess where the better player would basically always win against a weaker opponent. That's because your brain can react much faster on tactile information than on visual information.