Wales travel to Twickenham this Saturday for their tournament opener a fortnight after the Ospreys triumphed 17-12 over the reigning English Champions and current league leaders at the Liberty Stadium.
And although Williams admits that this weekend's international encounter will be an entirely different ball game, he does believe that the January 23 victory will stand Wales in good stead for the visit to English rugby's headquarters.
"There is a slight psychological advantage having beaten a very good English team from the Guinness Premiership," said Williams.
"We'll be confident going to Twickenham but we won't get carried away.
"Just because we beat Leicester does not mean we'll beat England. It will be a different story and a different animal."
Williams has also backed his Ospreys team-mate James Hook to shine in an unfamiliar role in a Welsh shirt.
Wales coach Warren Gatland has named Hook at outside centre and Williams believes his Ospreys colleague will quickly adapt to his new position.
Hook, who has largely played second fiddle to Dan Biggar at the Ospreys this season, has featured at fly-half, inside centre and full back for his country but has never worn the No13 shirt in international competition.
But that doesn't worry Williams as Wales begin their search for a third Grand Slam in six years.
"You can put James anywhere on the field," said Williams, who toured South Africa with the Lions alongside Hook last summer.
"He was almost thrown onto the field at 15 during the autumn and was probably our best player of the series.
"He's a tremendous player and one of the most naturally gifted players I've ever played alongside. He can create something out of nothing.
"I'm looking forward to playing outside him on the wing. He's never let the side down before and I know he will do a job for us."
Williams is known for his immensely quick feet and ability to create something from nothing but the 32-year-old admits that England have plenty of players with similar ability in their starting XV.
England team manager Martin Johnson has named the likes of Matthew Tait, Delon Armitage, Danny Care and Lions tourists Riki Fluety and Ugo Monye in an attack minded backline.
And former IRB World Player of the Year Williams is therefore expecting to have be at his best defensively if Wales are to contain a 'new look' England.
"England have selected a lot of quick players in the backline. In fact, they have some of the quickest in world rugby," added Williams.
"I'm sure they will be intent on running with the ball and scoring tries. We're expecting a really tough battle. But a lot will depend how the forwards play.
"England have a strong and physical pack and will aim to earn the right to get the ball wide and put the speed merchants in behind.
"You can't be one-dimensional and I'm sure they will try to use all their traits and weapons to break us down. But we'll be ready."
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