No cite from me without some searching, and I don’t recall one offhand, but I can vouch for many first round KO’s. Not too long ago, within the past year, ESPN2’s Friday Night Fights had several bouts decided within the first three minutes. This worked a real burden on their scheduling and they had to resort to a lot of filler and promos.
I can’t think of a reliable source for this info offhand, so I’m just waiting for a more efficient Googler to get you some one-punch stats. I’d be willing to bet we’ve had at least one this past 12 months – somewhere. Maybe not on TV, but somewhere.
Google showed me some (very impressive, I might add) one-punch knockouts, but they weren’t necessarily first-contact-of-the-fight; some were even in later rounds.
The fighters touch gloves, back up, maybe dance around a little (no contact), then BAM… down goes Fraaaa-zuh (or whomever).
K-1 rather than boxing, but Kid Yamamoto scored a one-hit knockout in under five seconds when he landed a knee right into his opponent’s chin. He throws a second punch before the ref stops the fight, but the guy was already down and out. This was 2005 or 2006.
Video:
His sister is also a top-level fighter, and smoking hot.