After assisting Woods to 13 major titles, Williams was on the bag when Scott won his only major - the 2013 Masters - during the three years they worked together. Williams said it would be "easy to jump back on the horse" but he's chosen a tough return venue because the US Open is at the Chambers Bay links in Washington, a new tournament course.
He has been enjoying the latest chapter in his life, including following nine-year-old son Jett's Kumeu rugby team, promoting and competing in motor racing, and working with his foundation which promotes junior golf.
"My intention was to have a complete break in 2015 and I haven't even been paying attention to the golf tour," Williams said.
"Have I missed the tour? Absolutely not after 36 years, and I have received a few approaches to go back.
"I had a few projects myself and was looking forward to having a break from all the travelling. I didn't expect the call (from Scott) and I was a bit shocked at first.
"But Adam is a very good player who performs well at the majors...and I like a challenge. I love the big tournaments."
Williams consulted his wife Kirsty and Jett about reuniting with Scott.
"I have been looking forward to following my son's rugby and I will have to miss some games now," said Williams.
"I asked him and he said it was definitely okay, so long as I didn't miss too many."
Scott said: "He took some convincing but I am very happy he's agreed to help out."
The Australian's brief partnership with Zimbabwean caddie Mike Kerr ended last week, and Scott told the Sydney Telegraph he had received all sorts applications from caddies since Williams retired.
"There were some shockers like random ones...some guy in Florida who lives at home with his mother," said Scott.
Scott's putting is a key problem and he has recorded 20 three-putts over nine tournaments. In a famous major moment, Williams over called Scott for the winning putt on the second play-off hole against Angel Cabrera at Augusta.
It was no small discrepancy, Scott seeing a one cup break, and Williams judging it at two-and-a-half. The warmth between the two was obvious in victory, and Scott has turned out to be a friend in need.
The significance is not only in the detail when it comes to Williams, who has caddied for the biggest names in golf - Woods, Scott, Greg Norman, Ian Baker-Finch and Ray Floyd who are all major winners.
His big personality can infuse golfers with all-important confidence. That might be the ingredient Scott needs most.
