Floyd Mayweather says he will fight just once more after next month's clash with Manny Pacquiao because he no longer enjoys the sport.
The unbeaten American's 2 May fight with Pacquiao is the fifth of a six-fight contract with broadcaster Showtime.
And Mayweather, 38, says the "fun" has gone out of boxing, which he now regards as "a job".
He added: "I don't enjoy it like I once did. My last fight is in September."
Mayweather took part in an open training session before telling reporters the Pacquiao bout at the MGM Hotel in Las Vegas would see two fighters "meeting at the pinnacle of our careers".
The fight is expected to be the richest in boxing history.
He added: "He's a future hall-of-famer. I am a future hall-of-famer."
WBA and WBC champion Mayweather is unbeaten in 47 professional fights, while his 36-year-old opponent has lost five and drawn two of his 57 fights.
After his defeat of Oscar De La Hoya in May 2007, Mayweather relinquished his WBC light-middleweight title, only to come out of his brief retirement in July to announce his fight with Britain's Ricky Hatton.
Mayweather also retired in 2008 but returned 21 months later in May 2009 to fight Juan Manuel Marquez.
The welterweight fight with Pacquiao has been the subject of on-off negotiations for six years and pundits say it will define who was the greatest fighter of the era.
Britain's Amir Khan has said he hopes to fight Mayweather - although he has also pledged to meet compatriot and IBF champion Kell Brook in the next 12 months and faces former light-welterweight world champion Chris Algieri on 30 May.
Former two-weight world champion Ricky Hatton - beaten by both Mayweather and Pacquiao in his career - says Khan's style would trouble Mayweather, if the Bolton boxer did become the American's final fight.
He said: "Amir is boxing with a lot more discipline lately, he's in and out and he's not getting involved. I think that type of style will cause Floyd massive problems.
"But, when you're the best, pound-for-pound, like Floyd Mayweather - and Amir's as big a name as is out there, he's beaten pretty much everyone - he might be forced to fight Amir Khan."