David Diaz believes the trials and tribulations of his life have given him a special resilience, and has warned next opponent Manny Pacquiao not to underestimate him.

Diaz knows from tough experience not to give up without a fight.

He tried it once and it didn't work out. So he came back, won the WBC lightweight title and became a pay-per-view champion of the world.

Diaz faces off against Filipino Pacquiao in Las Vegas on July 8, as Pacquiao targets a world title at a fourth weight.

Diaz is a major underdog. But the public can rest assured his days of giving up without a fight are long gone.

He repaired his fighting heart in his title contest against Jose Santa Cruz in Las Vegas in August 2006.

Heading into the 10th round, Diaz trailed by at least three points on all three scorecards. Only a knockout would suffice.

There was some kind of irony in the predicament he faced. A four-time national Golden Gloves champion and member of the 1996 US Olympic team, it was his lack of knockout power that had held him back, stopping him becoming a star.

After his first 13 fights, only five of which ended inside the distance, Diaz was unbeaten, but still unknown, and increasingly fed up.

A serious health issue for his mother tipped the balance. Fighting was the last thing on his mind.

"My mother had been on dialysis then she finally got a kidney transplant," said Diaz.

"After getting the kidney transplant, she had a heart attack. My being the small one in the family, I was the one helping out my mum.

"It took away from boxing. I lost track of what I was doing.

"I was a young kid, I didn't have my head on right. I had lost interest in boxing, gone away from it.

"I went back to working odd jobs - construction and stuff like that."

But there was to be no thoughts of quitting in Diaz second time around in the Thomas and Mack Center.

He knew the odds were stacked against him, but he kept pushing his taller opponent, seeking the shot to turn the fight in his favour.

It came dramatically midway through the 10th round.

A screwing left uppercut out of nowhere wobbled Santa Cruz and a follow-up combination dumped him on the canvas. Santa Cruz rose, blinking, on unsteady legs.

Diaz piled in to inflict more punishment. A succession of flinging hooks found their target.

Santa Cruz was down for a second time. He rose again, but referee Richard Steele stopped the contest.

After 10 years as a professional, including that two-year retirement and a potentially career-threatening eighth-round stoppage loss to Kendall Holt the previous year, Diaz had finally fought his way to the top.

The ultimate rewards for him would have been unthinkable when he was back working on the building sites, caring for his mother, seldom giving his stricken boxing career a second thought.

The next time the Chicago native stepped back in the ring, it was to face future Hall of Famer Erik Morales.

The Mexican was fading, but still expected to do a job on Diaz. But Diaz was undaunted. He won on points.

After a tune-up against Ramon Montano, the 31-year-old faces another pay-per-view test in Pacquiao. He knows there are few outside his immediate circle who give the blue-collar Diaz much of a chance.

But he believes the travails of his early career will stand him in good stead to produce another upset win. He is used to being written off, which is hardly surprising, because he got in the habit of doing it himself.

"Getting back into the sport after I'd stopped was kind of rough, and when I started boxing again I started to realise what I'd lost," said Diaz, whose first fight back was a sixth-round stoppage of Anthony Cobb in his home city.

"But people stuck by me and I went all the way to the title. Now the fight with Manny is the toughest fight of my career, but the kind of fight every fighter needs in order to prove he deserves to be the best.

"This is the chance for me to prove myself. I know a lot of the writers out there don't really believe in me. But I believe everything will come out good for me.

"I have the utmost respect for Manny, but I can't allow him to become champion."

The loss to Holt, in which he picked himself up in the first round and floored his opponent in the seventh before losing one round later, remains the only loss of a 36-fight record which once threatened not to amount to much.

But if he beats Pacquiao, Diaz will have another reason to relish his decision to return to his sport.

From construction worker to pay-per-view champion, 'never give up' is the motto which has sustained David Diaz every step of the way.

Sporting Life / Eurosport