Muhammad Ali's filmmaker daughter Hana insists new documentary I Am Ali will shock many of the boxing legend's fans, because it reveals her father was battling Parkinson's disease long before he retired from the sport.

The fighter, who has now been silenced by the condition, gave Hana the rights to all his recordings and urged her to sell them, but she decided to meticulously go through them with a view to piecing together a tribute to her dad.

And she admits it quickly became clear the sports legend was experiencing the early signs of Parkinson's years before he was diagnosed.

She tells Wenn, "I actually have recordings where you could hear his slurred speech and my mother and my grandmother say in the recordings, 'You're slurring your words, you should speak clear (sic)'. And he says, 'Oh yeah, I know. Sometimes I don't understand myself'.

"There are recordings with his lawyers talking about his worries and concerns; talking to his parents struggling with the decision to whether or not he should come back and hold off on retirement. Those were the hardest ones to hear knowing what would happen. He was so worried about retiring on top and so worried the world wouldn't love him. He's so humble even though he knows how important his legacy is."

Her sister Maryum adds, "I think retirement should have came (sic) after the Foreman fight (in 1974). Definitely after the second Leon Spinks fight. Most boxers don't know when to quit, but we saw what it was doing to my father and we could feel it."

Ali announced his retirement after the Spinks rematch in July, 1979, but he staged an ill-advised comeback to face Larry Holmes in an attempt to win the heavyweight championship an unprecedented fourth time.

Despite the signs that the legend was struggling with health issues following a thyroid cancer battle, he was cleared to fight Holmes and the bout took place on 2 October, 1980 in Las Vegas. Ali's trainer Angelo Dundee stopped the fight in the eleventh round and it became the only match the boxer lost by knockout.

Ali fought one last time in December, 1981 in Nassau against Trevor Berbick - he lost again.