One of the titans of 20th century popular culture, "The Greatest" was hardly a wallflower, and his oversized, well-documented life has sustained biographers, historians and filmmakers for decades.

As demonstrated by his sprawling, masterful "The Vietnam War" from 2017, Burns is not one to just acknowledge but relish in the context.

"He's the greatest athlete of the 20th century," Burns tells CNN.

"His life and his professional life intersected with all the main issues of the second half of the 20th century, that has to do obviously with sport and the role of sports in society and also race and politics and faith and Islam and war ...

He's just the most compelling figure in all of sports."

The director shares he has a neon sign in his editing suit that reads, "it's complicated." The same applies to Ali's life.

"There's no message" to the documentary, he insists, "we're in the history business."

"All of human life is complicated and contradictory and sometimes controversial, but there's a majesty to this particular life, and I don't think I've met an American as filled with the kind of spirit and sense of purpose as (Ali)," says Burns.

Walking through Ali's life is a well-trodden path for boxing fans, but by dint of the volume of material Burns had to hand there are still some surprises.

Ali said he'd fight the up and comer in two years if he kept on doing what he was doing and sent Frazier off with an autographed photo.

Ahead of his 1970 comeback fight against Jerry Quarry in Atlanta he was sent a box with a decapitated black dog inside and a note that read, "We know how to handle black draft-dodging dogs in Georgia." Ali's status as a conscientious objector would ultimately be vindicated when the Supreme Court unanimously overturned the conviction 8-0 in 1971.

When Ali lost to Frazier, Muammar Gaddafi declared a day of mourning

When Frazier won "The Fight of the Century" on March 8, 1971, Ali fans were distraught.

There will be a billion people watching." Ali's surprise appearance remains one of the defining images of any Olympic Games.

Ken Burns' "Muhammad Ali" debuts on September 19 on PBS.