The 19-year-old helped the Reds advance to the fourth round of the Carabao Cup by playing the entirety of Thursday night's 7-2 victory over Lincoln City.
It was a dream come true for the defender, having signed on with the club at the age of nine.
Asked about the moment he found out he'd be in Jürgen Klopp's starting XI, Williams told Liverpoolfc.com: "I don’t know how to sum it up.
"I'm nervous obviously but I'm over the moon because it's what I've been striving towards, all the hours I've put in.
"I got my reward and I'm buzzing, absolutely over the moon!
"I had an inkling two days before and then yesterday we did set-pieces and I was starting with Virg – that made me feel safe obviously. The day before I prepared right and we got the result.
"He [Klopp] told me not to be nervous and to enjoy myself and play with the boys because obviously stepping up here is a big thing for me, playing with world-class players.
"They helped me through it but he just told me, 'Don't be nervous and play your game.'
"It's a moment me and all my family dreamed of since I was a kid signing at nine.
"I've been rewarded with a new contract and now the manager has put his faith in me to play in a cup game, which I'm over the moon with.
"It's a brilliant result for the boys and happy to get through to the next round."
Williams was partnered by Virgil van Dijk for the first half of the tie before Fabinho then took over following the interval.
"You can see why he [Van Dijk] is the best in the world," Williams said. "He is talking constantly.
"He knew I was young and I was nervous. I've never felt more secure next to someone. Brilliant."
The Liverpool team on the night also contained Academy graduates Neco Williams and Curtis Jones, who claimed the Man of the Match award after a two-goal display.
Williams knows the challenge for him now is to continue impressing the Reds' coaching staff in order to claim further first-team opportunities.
He continued: "Neco and Curt are the two most recent ones obviously. They've managed to establish themselves now as proper players in the team.
"They did well last year and got in, played a few games in the Premier League and helped the team to win it.
"That's my next aim now, to try to become one of them boys and keep pushing on from there."