LAS VEGAS — In the industrial area behind Circus Circus, where tourists do not venture intentionally, is a squat concrete-block building that houses Barry’s Boxing Center. Inside, high on a wall crowded with framed photographs of boxers, familiar and not, is the defeated face of Floyd Mayweather Jr.

He is looking at his right hand while, on the other side of the referee, Augie Sanchez is forever caught in celebratory midleap. It was 1996, at the United States Olympic trials, the last fight that Mayweather admits that he lost fair and square.

“I never thought this guy, who I roomed with, who was 106 pounds when I met him, now he’s the best pound-for-pound fighter and going down as one of the best fighters in history?” Sanchez said Tuesday in the quiet lobby of the gym. “I would have never predicted it.”

Two miles west, journalists and fans swarmed Mayweather Boxing Club, in a suburban strip mall. Cars were towed to make room for the satellite television trucks. Tents were set up in the parking lot for catered food and a news conference. Bodyguards the size of door frames and wearing matching Mayweather T-shirts kept order among fans and loiterers with their mere presence and the occasional soft-spoken word. David Hasselhoff emerged from a car, and everyone surged toward him.