It's an oddity that possibly the most popular and unpopular players in New Zealand rugby have a friendship which cuts across national boundaries, team allegiance and hype around them.
Williams is almost as much a celebrity as a sportsman in New Zealand: an athletic giant who has played rugby league, rugby union and who boxes professionally. Cooper, though New Zealand-born, has become a pantomime villain to All Blacks fans for his perceived brashness and his antagonism towards New Zealand captain Richie McCaw.
Williams put in a plug for Quade Cooper the human being during a news conference on Wednesday. He told reporters he kept in touch with Cooper and liked him but said mateship would count for nothing when the All Blacks and Wallabies meet in the World Cup semifinals on Sunday.
He also sees New Zealand's "hatred" of Cooper as a good sign, a mark of respect for a player of extraordinary talent.
"I've been in touch with him, he's a good mate. But on the field it's a different story," Williams said. "I want to win and he wants to win, I want to win for New Zealand, he wants to win for Australia.
"Outside of the field we're really good mates and that's how it will always be.
"I think with Quade coming over here, the spotlight's been on him. Obviously with us being a proud rugby nation we see him as the enemy, so to speak. But I think he's handled himself pretty well.
"I think it's kind of a sign of respect as well because when people are hating on him that much it just shows how good a talent he is."
Cooper has equally been supportive of Williams thoughout the World Cup and expressed similar admiration of his ability.
"I've sent him a couple of texts, wishing him well, and enjoying the way that he's been playing," Cooper said. "He's showed through the World Cup he's a very talented player, a very special player. Every game that he's played he's been moved around, and each game he has excelled. it just shows what a talented athlete he is."
Williams copes with his own celebrity as easily as he dismisses Cooper's notoriety. When the All Black team has been introduced to fans before World Cup matches, their largest cheer has often been reserved for Williams. When he receives the ball in any match, a ripple of anticipation passes through the crowd.
Asked if he felt the pressure of fans' expectations, particularly heading into Sunday's semifinal, Williams said "no, not really.
"I think I'm just doing what I usually do, not reading media. Just doing all the little things right, ticking all the boxes during the week and staying in the now, not looking back at the past - what's happened - or at the future - what might happen.
"I'm just really focusing on the detail parts of my game and, like I said, just ticking those boxes and I just feel sweet going into the game knowing I've done all I can.
"Sometimes you're going to play good, sometimes you're going to play bad but at the end of the day if you ticked all those boxes it's sweet."
Williams' selection in the All Blacks' semifinal lineup is far from certain. He's played recently as a winger, because of injuries, but he may not be able to hold that position with Richard Kahui now fully fit.
New Zealand is unlikely to break up its most experienced centre pairing of Ma'a Nonu and Conrad Smith, and his chance of winning a place on the bench may depend on other contingencies.
The All Blacks are likely to carry cover at scrumhalf and flyhalf and a versatile player to cover the outside backs roles. Williams has immense value as an impact player but isn't as versatile as Isaia Toeava.
"Firstly, just hopefully I get picked and I'm involved," he said. "That's probably the first thing and if I am it's going to be great.
"There's nothing better than this stage, (it's) probably the reason I came back here, to try and test myself against the best players, to try and make this team, make this squad and hopefully we can do the business.
"(Australia) are a good side and I think they're coming into form at the right time. I think we haven't played our best footy as well. It's going to be a great match and I just want to be involved bro."
Williams' move to winger was more successful than even he expected.
"I think I've surprised myself how well I went on the wing," he said. "I didn't think I could do it but some people probably thought I couldn't play rugby and I've showed them that I can. So I'm always a confident player and if I'm put out there I know I'll do the job the best that I can."
Williams has a sharp wit but his humor comes as well from an unguarded part of his nature, his willingness to say what he's thinking.
The current All Black team lunched this week with the 1987 team, the only New Zealand team to have won the World Cup and one of New Zealand's most respected sporting sides. Asked what he had taken from that meeting, the chance to talk with some of the greats of the game, Williams replied "just seeing that we all get old at some stage."
At that Williams' teammate, the 36-year-old lock Brad Thorn, burst into uncontrollable laughter.
"I'm not having a go at them," Williams said when he realized his comment might be misinterpreted. "We all get old at one stage but they can still look back in 24 years' time and say 'yep, we won a World Cup.'
"If we can do that we'll probably be doing the same thing and that's probably the biggest thing."