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Williams, counting down his final days in mandatory quarantine in a Sydney apartment before joining his Roosters teammates, said it was hard to believe he was on the verge of an NRL comeback.
The 35-year-old former All Blacks and Roosters premiership player has signed a short-term NRL deal.
His return to the NRL comes after a stint with Toronto was cut short because of the pandemic that has forced the Wolfpack to withdraw from the remainder of the 2020 Super League season.
"I'm not going to lie. I didn't know when the Toronto thing fell through. I was kind of tossing up if that was it," Williams told Nine Network's 100% Footy.
"I was a little bit sad to think things might have ended on that note.
"But then you fast forward a couple of weeks and the opportunity arose to come back here.
"And it's like 'well, let's give it a crack and see if I can still do it'.
"I'm 35 coming off surgery so that's the challenge ... hopefully we can make some good new memories over the next few months."
Williams rejoins the Roosters as they bid for their third successive title amid one of the worst injury tolls in the NRL.
He will finish quarantine later this week and is reportedly on a timetable to be available for selection when the Roosters meet Canberra in another grand final rematch in the nation's capital on September 5.
After starting his NRL career with Canterbury, Williams walked out on the Bulldogs in 2008 to join French rugby side Toulon.
He went on to enjoy a successful Super Rugby career as well earning 58 caps for the All Blacks, winning a World Cup, before he was lured back to rugby league.
Williams made an immediate impact on his return to the NRL in 2013, helping the Roosters to premiership success.
He stayed on for the 2014 season before returning to rugby.
Williams, meanwhile, said a boxing bout with retired NRL star Paul Gallen was "one hundred per cent" certain to happen.
According to Williams, the fight could be held within six months.
Gallen has long held an ambition to get into the ring with Williams but their boxing careers are yet to cross paths.