LAS VEGAS - Floyd Mayweather may insist Saturday is his last fight in a stellar career, but his welterweight rivals are hoping they will still get the chance to dethrone the pound-for-pound king. And they warned that the Las Vegas showdown against massive underdog Andre Berto just might not be the cakewalk that many fans and pundits have predicted. Mayweather, 38, faces Berto -- who has three losses in his last six fights -- at the MGM Grand Garden Arena striving to equal the perfect 49-0 record of heavyweight legend Rocky Marciano. The unbeaten American insists it will be his swan song, the crowning moment of a glittering 19-year career as the best boxer of his generation. But critics accuse him of bigging up the retirement talk to flog a fight that barely 24 hours away had failed to sell out.