The 37-year-old tells The Straits Times and other press in Los Angeles that he "made so many sacrifices and turned down so much business" to do this film, in which he plays the fabled boxer, Sugar Ray Leonard, who is not even the lead character.

Opening in Singapore tomorrow, Hands Of Stone - based on the book of the same name by Christian Giudice - is about Panamanian boxer Roberto Duran's two fabled 1980 fights against Leonard.

Usher, whose full name is Usher Raymond IV, says it took more than two years to complete the movie because of delays, changes in location and casting, and the difficulties of finding financing for independent films.

By the time everything came together on the production, the juggernaut of the eight-time Grammy winner's music career - which has seen him churn out hits such as Without You (2011), Love In This Club (2008), U Got It Bad (2001) and the recent single, Crash - had already moved on, and the star had other obligations.

"But I still wanted to do the film. So I turned down tours, I turned down releasing an album, I turned down going back to The Voice, which I enjoyed," he says, referring to the reality television show and singing competition where he was a judge in Seasons 4 and 6.

"I went on a small tour domestically and internationally, but had to come right back home, do my final season of The Voice and then go to film," says the star, who will release his long-awaited eighth album, Flawed, later this year.

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The opportunity to play a sporting icon was too good to pass up, especially once Leonard, now 60, agreed to help.

"I wanted to get his blessing before I took it on. I said, 'Sugar, they asked me to play you. I just want to get it right - are you okay with that? If you don't want me to do it, I won't.'

"He said, 'Man, my wife is a huge fan of yours. She loves you more than she loves me.'

"I said, 'Cool. Can I sit with you? Will you be available when I need to ask you questions?' He said, 'Of course, whatever you need to do - come to the house and I'll come watch you box and answer any questions you need.' He made himself available."

The first match between Duran and Leonard ended with the underdog Duran scoring an upset over Leonard, then the welterweight champion.

When the two met for a rematch five months later, the Panamanian stunned his fans and trainer Ray Arcel (De Niro) by quitting mid-fight, uttering the now-immortal words "no mas", or "no more" in Spanish.

Usher had to get in serious shape, training with boxing coaches and amateur fighters and also working to lose more than 6kg to get closer to Leonard's fighting weight at the time.

"I wanted to stand toe-to-toe with amateur boxers in the ring because I didn't just want to do it based on the choreography that was given to me - I wanted to know what type of pressures these types of athletes go through when they're standing face to face with an opponent who's trying to cause them harm.

"So I would do three-minute sparring rounds and I wouldn't hide. And I'd do six to seven rounds," he says.

A little easier to nail down was Leonard's classic 1980s hairdo.

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"Sugar Ray said, 'You've got to have a real good afro - like, pick it out all the way. So I grew it out for a year until I had a big old afro," says the singer, who usually sports a close crop.

"I loved every aspect of the preparation and getting to know the character. I was probably in the best physical shape of my life and have been kind of slowly declining from that moment," he adds wistfully.

The film's protagonist is Duran, played by Edgar Ramirez, but Usher felt it was just as crucial to honor Leonard, who was a powerful symbol for the black community.

"I felt fortunate to be able to represent an icon to a lot of African- American people because of what he represented - not just as a boxer, but also as a symbol for what we could become, as an African American who was an Olympic medalist (in 1976) and who was sought after by major corporations to endorse their products.

"That was a major accomplishment for an African-American person," says the singer, who is married to his manager Grace Miguel, 47, and has two sons, aged eight and seven, with ex-wife Tameka Foster.

The performer - who appeared in films such as The Faculty (1998) and television series such as Moesha (1997-1999) - believes the time to pay tribute to such figures is when they are still alive.

"It's great that we have had icons who have been able to see their legacy before they die. (Boxer) Muhammad Ali had that. (Musician) Miles Davis didn't necessarily have that. Ray Charles, he got a chance to feel that. And now, Sugar Ray Leonard will get a chance to feel that."

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Usher adds that the movie will show a different side of Leonard from his popular image as "a guy who was flashy and had the boy-like smile" and also show that his rivalry with Duran was partly the result of Leonard standing up for his wife, whom Duran had insulted.

He also feels it is his duty as an African-American to make sure stories such as these are told.

"As a culture, you have all these regions of people who have been displaced because of the African diaspora and our culture and voice have been taken. I speak English, I don't use an African language or any type of dialect that is my own and I don't understand or know what my original religion is. So, this is the only way we get a chance to preserve the legacy of the hard work that went into creating our culture."

While there have been many books and articles written about these men, he says there is something special about immortalizing them on film.

"That sort of interpretation is burned into your mind. I'll never forget Denzel Washington in the movie, Malcolm X (1992), or Don Cheadle playing Miles Davis in Miles Ahead (2015).

"So, I approached this with that in mind: I wanted to make sure I get this right for the sake of our icons because they're important."