The reigning world men's sevens player of the year is still haunted by the unsolved murder of his childhood best friend, a tragedy that helped propel him to the top of the global game and drives him still as he plots a life beyond rugby and a career in criminal investigation.

Baker was a basketball-obsessed eight-year-old when the murder of his best mate Dimitric Moore turned his life and the lives of the New Smyrna Beach, Florida, community upside down.

The two boys had spent a Sunday afternoon riding their bikes around the neighbourhood, feeding horses, playing basketball and video games, before going their separate ways at dusk on April 23, 1995.

Baker was the last person to see Moore alive, a circumstance that put him in the witness stand when Moore's mother was tried and later acquitted for manslaughter.

"When the streetlights come on, my parents used to say it was time to go home," he said.

"We were out riding bikes, there were four or five of us, playing basketball ... we were feeding a horse right around the corner from my grandma's house, then we went back to my grandma's house and when the streetlights came on my grandma said 'it's time for you to come in and you guys can go home'. He only lived four or five blocks from me."

Moore hopped on his bike and rode away.

"I will never forget it, I can still see it to this day," Baker said. "Going to school and he's not there, the next day he's not at school, they come out with these flyers on the third day and I was like 'what, where has he been?' and people said he was missing, everyone's freaking out."

Moore's body was found in the boot of his mother's car and the investigation and court case that ensued left a lasting impression on Baker.

"After that, in my head, I wanted to be a police officer. I just wanted to catch bad guys," he said. "As I got older I realised that's not a police officer, that's a homicide detective. That's why I went into the field I went into."

A talented high school footballer, Baker studied criminal justice on scholarship at Fairmont State University in West Virginia. He had already been introduced to rugby - by his high school football coach, ironically - and continued to dabble in the sevens format even as he tried to crack the NFL. An undiagnosed knee injury brought that chapter to a close just as he was close to signing with the Philadelphia Eagles in 2011. He tried to play his way back via the Arena Football League but by 2013 viewed rugby as his best chance of a professional sporting career.

The challenges kept up. Naysayers said Baker was too small and too late to master rugby, but he worked nights as a campus security guard to put himself through a private rugby academy in Ohio, while friends back home staged fundraisers to help keep the dream alive. Next came a stint selling sports therapy goods and then work as a pest controller. He had just sprayed a house when the call came through that changed everything.

"They said they wanted to offer me a contract, I just lost it," he said. "I'm in the truck, bawling my eyes out. I called my parents straight away and then had to move to California by the weekend."

That was July 2014 and three months later he made his world series debut on the Gold Coast. In the three seasons that followed Baker and his teammates helped transform the USA Eagles from the minnows of world rugby to giant killers, capable of knocking off the traditional powerhouses. A season-topping haul of 57 tries and 285 points last year and an electric partnership with track star Carlin Isles helped the US finish fifth in the world and earn Baker the world player of the year award.

Now 31, the 185cm juggernaut they nicknamed "speed stick" has two things on his mind: helping the US shed the giant killer tag and regularly crack the top three on the sevens circuit ahead of the first Sevens World Cup to be held on US soil. The San Francisco tournament is targeting 100,000 tickets across three days in July this year and is already sitting around the 60,000 mark with five months to go.

Baker will also figure out how he can make a difference off the field when the rugby dream finishes. He interned with the New York Police Department's homicide squad over the northern hemisphere summer last year, shadowed officers in the Portsmouth Police Department and sat in on divorce cases at a law firm in New Hampshire.

In Sydney playing in the third leg of the world series, though, his thoughts are squarely on how handsomely the hard yards have paid off.

"This is it for me, I just want to give it my all," he said. "This is for all the people who helped me along the way, who believed in me, raised money for me."

It might also be for eight-year-old Perry, who learnt about the darkness in the world a good few years too early. And for Dimitric, the little boy who never made it home.

"He was really really fast, that's why we became friends," Baker said. "He hit me and ran and I couldn't catch him, and we became best friends."