When Sonny Bill Williams retired from rugby union after the 2019 World Cup, he brought an end to one of the most glittering careers in the sport.

On top of a succesful time in rugby league Williams won countless Bledisloe Cups as well as the Webb Ellis Cup in 2011 and four years later.

On Friday, Williams' New Zealand play hosts France in the World Cup's opening game less than two months after Les Bleus' under 20s side won a third straight junior title.

"When I see the French, the chickens are coming home to roost," former centre Williams told AFP.

"The last few under 20 World Cups they've won and the young fearless players that are coming through, France represent that.

"They don't have the fear factor of the All Blacks, that's why they're scary, that's why for me they're one of the favourites," he said, adding Ireland to the list of title contenders.

The 38-year-old was part of an All Blacks generation to lift the Webb Ellis Cup twice, alongside the likes of Richie McCaw, Dan Carter and Ma'a Nonu.

"I was lucky, I was very fortunate to be in a great side," Williams, who spent two years of his union career with French club Toulon, said.

"I always loved the fact that I was in a group, in the sheds and I understood there were some great players in the team and all I had to do was my job.

"Some good memories, but everyone has their time," he added.

'KHABIB UPBRINGING'

The build-up to the tournament has been overshadowed by a group of former internationals pursuing legal action against World Rugby, England's Rugby Football Union and the Welsh Rugby Union for the brain injuries they suffered during their careers.

Williams, who has also had a successful boxing career, worries the sport is losing the next generation of players due to the issue.

"We want mothers to believe in our beautiful sport," practising Muslim Williams said.

"There need to be strategies in place that allow the players to get the best out of themselves by safeguarding themselves and being efficient in that space too.

"Knowledge through science and getting that hands-on approach from people that have actually walked that path," he added.

This weekend, the All Blacks start their attempt at a record fourth World Cup, by facing the in-form Les Bleus.

Captaining the home side will be Antoine Dupont, a nuggetty scrum-half who grew up near the Pyrenees mountains.

Williams compared Dupont to former UFC great Khabib Nurmagomedov, from Dagestan, Russia

"He's a special player," Williams said.

"He's had that Khabib upbringing, natural physical ability from being in fresh air.

"I like his style, very physical.

"He's a representation of this young scary French player coming through the French system, not scared," Williams added.