Roberts hobbled off with a hip injury 18 minutes into the 33-10 defeat to New Zealand and will not train until late in the week.
But Williams impressed from the bench, as he formed an all Scarlets midfield partnership and grabbed a try from Wales’ 13-man lineout.
And he reckons it is vital that Wales build upon their confidence to claim a long-awaited victory over the Wallabies.
“It’s massively important for us to try and get a win – but I think we perform better under pressure,” said Williams.
“We must take the confidence we got from the last half-hour against New Zealand into the game and take it to Australia in the opening 20 minutes.
“One of our goals is to avoid an autumn whitewash. Obviously this weekend is massive for us – we have to get a win.
“It will probably be the same against them as in the summer.
“They have got some good individuals, they play quite an open game, with the same high intensity as last Saturday. We have to live with that.
“Their back-three look sharp and they like to run in the No.9 and 10 channel. They’re all pretty good ball players but we will be looking to nullify them.
“They’ll be quite similar, although maybe not quite as good as New Zealand.”
But Williams insists Wales must learn from their lesson at the hands of the All Blacks and begin the Millennium Stadium clash firing on all cylinders.
“Southern hemisphere teams tend to play at a much higher intensity but it was up another step against New Zealand,” said the 22-year-old.
“We need to take that intensity into the match with Australia.”
“As a team, it’s how you react to losses. If we learn from this experience, I’m sure it will make us better players and a better team.”
Tickets are still available for the game with Australia priced at £60 from the WRU Ticket Store (St David’s Dewi Sant), via Ticketmaster (08442 777888 and www.ticketmaster.com) and Ticketline (02920 230130 and www.ticketlineuk.com).
