The 29-year-old flanker has experienced some of Welsh rugby's darkest days, most notably an embarrassing Six Nations whitewash just two years ago.
So Williams can be excused if he feels like pinching himself as the magnitude of tomorrow's Millennium Stadium clash against Ireland sinks in, with Grand Slam, Triple Crown and Six Nations title honours all at stake.
"If I am totally honest, I never thought I would be involved in a Grand Slam-deciding game for us," said the Cardiff Blues forward, whose magnificent performances this season have made him almost a certainty to tour with the Lions in the summer.
"It is a dream come true, and I am trying to say to the younger players that they should savour it, not be overawed by it.
"Two years ago, you couldn't really see much hope, so to speak. We thought it would perhaps take five or six years, but we've worked hard and pulled together.
"If someone had said to us two years ago we would be in a Grand Slam game in two years' time, we would have laughed at them because we didn't win a game. It is an added bonus to be part of it all.
"As a player, you always want to play in big games, and it is just great to be involved in something as big as this."