Danny Wilson's young side have beaten Samoa, Scotland, Argentina and South Africa on their way to becoming the first Welsh team to reach a JWC final, and Scarlets scrum-half Williams believes some of that edge comes from their only defeat in the Six Nations - to England.
Williams said: "It is a brilliant achievement to reach the final but we are already very focussed on that next challenge which is to win on Sunday.
"We are not carried away with our progress to date. We had one day of recovery and then we were straight back into our analysis and preparation for the final.
"We are not over-confident - England beat us convincingly in the Six Nations so we won't be getting ahead of ourselves.
"However, we have shown in this tournament that we have the determination to dig in right to the end and win matches. We are going to need at least the same amount of commitment to beat England on Sunday.
"When we lost to England in the Six Nations, we were distraught and as Ellis (captain Ellis Jenkins) has said throughout this campaign, we don't want to feel like that again. That thought will give us even more motivation to win on Sunday.
"Without taking anything away from England - they were excellent on the night - we didn't get certain things right when we played them in the Six Nations. We have built as a group during this tournament and shown we can adapt to different styles of opposition.
"Our defence has also been good, which will be important on Sunday."
Williams says the side will be helped on Sunday by some extra Welsh support.
"Support from back home has been brilliant. Some families haven't been able to come out until now but a lot more are coming for the final so that will give the boys an extra boost," he added.
"England are a great side, they are very fast and physical and they stopped New Zealand playing out wide in the first half. It should be a really good game on Sunday, we can't wait."