The Cardiff Blues centre is one of four players making their senior international debuts alongside Dafydd Howells, James King and Emyr Phillips.
And the Aberdare product is determined to savour every moment of the landmark occasion.
“We were in a team meeting and to see my name on the screen was amazing,” said Williams.
“It’s very exciting and I’m really looking for to winning my first cap. I’m sure I’m going to be nervous, but I want to enjoy it and remember every moment of it.
“I couldn’t ring home straight away because it was the middle of the night and we were going straight to training. So I had to wait to ring, but now my parents are coming out to watch the game.”
Williams enjoyed a breakthrough season for the Blues, widely impressing with 20 appearances in the midfield and on the wing for the region.
And the 21-year-old is determined to take centre stage as Wales bid to continue the form of their triumphant RBS 6 Nations campaign.
“This has been a breakthrough year for me,” said Williams.
“I’ve had a lot of game time with the Blues and I’ve been happy with the year I’ve had.
“I played a lot on the wing because of injuries, but this (No. 13) is my best position.
“It has been great to get some games under the belt and I’m already looking forward to next season. The Blues are going to be a lot stronger because a lot of youngsters like myself who had just stepped in this season have now had a lot more experience so should be better for it.
“This (first cap) is the perfect way to finish off a good year.
“It’s a chance to put your hand up and put your name in their heads for when they pick the next few squads.”
While Williams is taking his first steps into the Test arena in Japan, his regional teammate, Racing Metro-bound Jamie Roberts is starring for the Lions.
And Williams admits he has learned a huge amount from the British & Irish Lions mane man.
“It helps a lot with the experience he has got,” added Williams.
“He helps you along, talking a lot through the game and it just lifts you because you’re playing alongside him. He gives you confidence.”
