MIAMI -- By most accounts, undefeated boxing champion Floyd Mayweather Jr. delivered a knockout message Sunday to the Miami Heat as they set out to become just the fourth team in NBA history to win three straight titles.

Mayweather was seen exiting the Heat's practice facility just as reporters were entering to interview players. By then, he had already gotten a power message across to an awed LeBron James and several of his teammates.

"With him being 17 years-and-0 -- we're trying to defend two titles -- he's been defending for a long time," James said. "He definitely knows where we're coming from. It's definitely exciting to have someone, one of the greatest of all time, to be in your presence. It's something you can talk about years from now when you look back on the history of sports. That guy was at our practice."

Floyd Mayweather Jr. talked to the Heat about remaining on top in the face of what is expected to be stiffer competition, players said Sunday. Jayne Kamin-Oncea/USA TODAY Sports

Heat players said Mayweather, who is 45-0 and has won titles in five different weight divisions, talked to the team about remaining on top in the face of what is expected to be stiffer competition from much-improved contenders across the league.

Miami spent Sunday preparing for its preseason opener Monday against Atlanta, and the two-time defending champions open the regular season Oct. 29 against Chicago. The Heat are trying to become just the fourth team in NBA history to win three consecutive titles.

Mayweather, 36, has collected at least eight major championship belts over his decorated career. He has attended several Heat games over the past three seasons and has fought at least once at AmericanAirlines arena during a 17-year boxing career that has made him one of the wealthiest prizefighters in history.

But it's rare moment when Heat team president Pat Riley, known for running a tight-knit organization, has allowed outsiders -- even high-profile celebrities -- to attend workouts and address the team.

Riley, however, is also a huge boxing fan and has followed the sport since before his playing days in the NBA. Heat star Dwyane Wade, who has been in Miami for all 11 of his NBA seasons, said he didn't recall a similar practice visit from another icon.