And Williams, who has aiming to play in a third World Cup, knows this is the time to shine if you want to make the final 30-man squad.
"These games are vital," he said.
"It's a chance for people to put their hands up and show that they want to be involved.
"It's a nervous time, its been a long hard season and everyone has trained hard to get to this point and now is the time to shine.
"The competition for places in the final squad is fierce and that is good for the individuals and the team as a collective.
"There are a lot of excited boys looking forward to going to their first World Cup and others aiming for their third or even fourth tournament.
"But there certainly is no complacency in the squad, no-one is assuming they are going to get picked. The youngsters are impressing and it keeps everyone on their toes, which is great for Welsh rugby.
"You know one slip-up, or if the guy in your position has a great game and you could be sitting back home when the World Cup is going on in New Zealand."
You can follow Wales' progress minute by minute at Twickenham by logging onto http://www.wru.co.uk/live just before kick off.