Freddie Roach, pictured here in Hollywood on February 17, 2015, has had a long career as a cornerman training some of the world's greatest boxers © Getty Images/AFP/File Jonathan Moore Macau (Macau) (AFP) - Hall of Fame trainer Freddie Roach, who turned 55 on Thursday, has seen it all during a legendary career as a cornerman to the world's greatest fighters.

But he told AFP Thursday that training Manny Pacquiao to face the unbeaten Floyd Mayweather in boxing's most lucrative fight yet will be "the biggest challenge of my life".

Roach has trained a string of world champions such as Bernard Hopkins, Julio Cesar Chavez and Oscar De La Hoya. He has been in Pacquiao's corner for 15 years, been voted US trainer of the year seven times, and endures a constant battle against Parkinson's disease.

Yet he still insisted in an exclusive interview that beating Mayweather would top all that.

"It's the biggest challenge of my career, by far," the American said in Macau, where he is preparing China's Zou Shiming to challenge Thailand's Amnat Ruenroeng for the IBF flyweight championship at Cotai Arena on Saturday.

"Floyd's undefeated. He's talented, He's unique. Here's a guy I first saw when he was five years old in the gym. And he was a good fighter then," Roach said.

The unbeaten welterweight Mayweather (47-0, 26KOs) faces eight-division champion Pacquiao (57-5-2, 38KOs) in Las Vegas on May 2 in the richest fight of all time.

"He's trained himself his whole life. He's a natural fighter and he makes the moves himself, I mean Floyd is a self-made fighter. He was born into this sport. He just grew with it.

"I'm happy this fight is here. Yes, it's the biggest challenge of my life but I think it's going to be the greatest moment of my life also."

Roach spoke to AFP straight after getting off the phone to Los Angeles, when he had received a special greeting.

"I just talked to Manny," said Roach. "He wanted to call me. He sang 'Happy Birthday' and he had the whole gym singing to me!"

Roach said Pacquiao could beat Mayweather but cautioned it would take a near-perfect performance and supreme concentration from 'Pacman'.

- 'No room for error' -

"The strategy to win the fight is very complicated and very precise," said Roach. "Manny has to make the right moves at the right time to win the fight and this time there is no room for error.

"Mayweather is very good at moving and when he gets you to follow him he will walk you into shots, score points and win rounds.

"But he does make mistakes here and there. Not a lot, but a couple and that's something a southpaw like Manny can take advantage of."

Roach clearly admires the ringcraft of Mayweather, but said the man known as 'Money' left a lot to be desired as a person.

"I think Manny will do the world a public service to beat this guy because he's not a good role model," said Roach.

"Manny's a really good role model -- he's someone I would want my kids to look up to."

- Stay or go? -

Pacquiao, who has carved out a parallel career in politics, arrived in training camp at Roach's Wildcard gym in Los Angeles last Saturday, just as Roach was about to leave for China with Zou.

Roach revealed his first thoughts were to stay behind to help 'Pacman' prepare for Mayweather, but the Filipino superstar was having none of it.

"He came to camp early because the Philippines is crazy," said Roach of the media and fan circus that follows the adored Pacquiao around.

"So I said to him: 'I'll see you on Monday'.

"And Manny said: 'No, I'll see you when you get home from Shiming's fight. Go help Shiming win the world title. Because with you there he has a much better chance of winning. I can train myself for five days. I'm OK'.

"I think I should be with Manny who's got the biggest fight of his life coming up but so has Zou. Not too many guys would do that but Manny's such a nice person."

Roach directed a blast at Mayweather's father, Floyd Snr, who will be in the opposite corner come May 2.

Roach believes Mayweather would be better off with his uncle, Roger Mayweather, as his trainer.

"He was a really good fighter at five and I saw him grow up with his uncle Roger. I'm kind of happy that Roger is not the trainer any more because the father isn't as good," said Roach.

"The father gets so excited he can't even talk in between rounds sometimes. Roger's been there 90 percent of the time but he chose to have his father in the corner so I'm happy about that because I don't think his dad's that good."