September 17th, 2011. He KO'd Victor Ortiz in the 4th round.

IMO, Mayweather just wants to win a fight, he doesn't chase the KOs. I think his desire is to beat Rocky Marciano's undefeated record (he was 49-0). I have read a few articles that he may retire after this fight, but I doubt it. Right now his record is 45-0. Assuming he wins tonight, he will be 46-0 and will fight four more times then quit (and make a gazillion dollars in the process).