But Williams played a super-sub role and scored the famous match-winning try at Twickenham as Wales clinched the Triple Crown.
Now, with Roberts ruled out with a knee injury, Williams hopes to replicate his Triple Crown form from the outset against Australia.
"I'm just really happy to have the opportunity to start a game," said the 21-year-old. "I've played a lot coming off the bench, with Jamie being in great form.
"This is my opportunity now and I will do my best to try and take it. It's my big chance on the weekend."
Williams, who will win his 13th cap in Brisbane, was selected ahead of James Hook and Ashley Beck. He will form an all-Scarlets midfield with Rhys Priestland at fly-half and Jon Davies at outside centre.
He reckons the familiarity will be a major advantage but insists Wales face a daunting challenge against a wounded
"I think it's an advantage because we've trained and played a lot together," added Williams. "We are used to each other.
"We know we are going to have our work cut out. Australia are not going to be a pushover. They had a really scrappy game against Scotland and were unlucky there.
"But we are just concentrating on what we are taking into the game now. We have got a massive opportunity to put a stamp on a southern hemisphere side."