The Toffees duo are currently away on international duty, with a crunch 2018 World Cup qualification game to come on Friday.
Coleman’s Republic of Ireland are top of UEFA Group D after four rounds, and now host Wales – captained by Williams – at the Aviva Stadium.
Ahead of that meeting, Williams has identified his teammate as a key threat, having been left impressed since his summer transfer from Swansea City.
The imposing centre-back said, as reported by Everton’s official website: “He’s unbelievable, one of the best players I’ve ever played with.
“A right-back who can defend excellently but he’s also an attacking player – you see that with Everton week in, week out. He’s someone we’ll need to stop as he’s a very important player.
“We’ve had a little bit of banter already over the breakfast table! Normally we’re on the same team but now we’re coming up against each other, it’ll be fun.
“He’s very professional, Seamus, and he’s always fully focused. It’s an important game for both of us, we both need to win. It’ll be a difficult game so, like he will be, I’ll be fully focused on that night.”
Is Coleman effective at both ends of the field?
The full-back has impressively been outscored by only Romelu Lukaku for Everton this season, with four Premier League goals.
Coleman has also registered two assists, while his tally of 15 chances created is only two fewer than his 2015/16 total.
In a defensive sense, only Williams (21) has made more blocks than Coleman (16) across his 25 league appearances this term – and the latter has won the most tackles (43) of any Toffees defender.