Wales ensured their place in the last eight of the biggest competition in world rugby with a 27-15 victory over Six Nations rivals Italy at Canberra Stadium on Saturday.
But there was little time to revel in the glory with the prospect of taking on the two favourites for the William Webb Ellis trophy in successive weeks.
Wales face New Zealand in Sydney next Sunday before a quarter-final, expected to be against England, in Brisbane seven days later.
"The celebrations didn't go on for very long, just one drink and then bed as we were up early for bike ride around the lake," said full-back Williams after returning from a 45-minute cycle around Lake Burley Griffin in the heart of Canberra.
"Only half the job has been done though it's a great feeling to have qualified for the quarter-finals with a game to spare.
"There was a lot of pressure on us but it only really hit home after speaking with the media and the press and realising how much attention was on that one game.
"We haven't played as well as we wanted this season so it's nice to get a run of victories under our belt.
"But it would be easy for us to sit back and say that we have achieved our first aim and done what we wanted.
"We have two more games to go and they are against probably the two best teams in the world. But we can't worry about how good they are because then you perform badly.
"We have got to be excited by the challenge and remain positive and keep attacking. We do not want to be humiliated."