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“One of my best friends, maybe my best friend,” is how Jurgen Klopp described David Wagner this week.

Tomorrow, the pals will find themselves in opposite dugouts as Liverpool visit Huddersfield for a pre-season friendly.

It promises to be a convivial evening at the John Smith’s Stadium. The teams will be playing for the Bill Shankly Trophy, with the great man honoured by both sets of supporters prior to kick off.

In the stands will be Ian Ayre, once chairman of Town and now Liverpool’s chief executive, and Stuart Webber, the former Reds scout who now acts as Huddersfield’s head of football operations. Mike Marsh, a former Anfield player and coach, will be in the home side’s dugout.

And in the technical area, Klopp will go head-to-head with his the guy who stole his place at Mainz, who acted as one of his chief lieutenants at Borussia Dortmund, and who got so drunk at his wedding that he can’t remember his best man’s speech.

Klopp and Wagner met at Mainz in 1991. “He was a striker and I was a striker,” remembers Wagner. “He was the main one when I signed from Eintracht Frankfurt. Then I took his place and he sat on the bench — this was the start of our relationship.

“After a few months he dropped back as a defender, that made things easier for us!”

Teammates, roommates even, for four years - “sometimes I spent more nights with him than with my wife” jokes Wagner- the pair struck up a strong bond, which has lasted despite modern football’s itinerant nature – and Klopp’s bad habits.

“He doesn’t snore but he smoked,” Wagner says. “Although I only allowed him to smoke in the toilet, unless there was a balcony!”

On Tuesday, Klopp shared a picture of the pair from their Mainz days, showing the Reds boss in a bar with a cigar hanging from his mouth.

“When I was in Munich I remember playing against 1860,” he says. “It was a draw and it was one of my best games for Mainz. Afterwards we went to Oktoberfest, so it was a good night!”

The pair were reunited in a working sense in 2011 when Wagner, a German-born American international, was appointed manager of Borussia Dortmund II.

There, he would help develop the likes of Erik Durm for Klopp’s senior side, the pair regularly meeting for lengthy, animated chats on football’s intricacies.

“I am dark, he is light. He is tall, I am shorter. But our characters are very similar,” says Wagner. “We like the same jokes and doing the same things.

“We love exactly the same style of football. We’re very honest with each other. We speak several times a week - honestly and sometimes very harshly.

“Of course I’ll watch Liverpool on TV and criticise him. He knows I don’t have to say something he’d like to hear. I say what I recognise.”

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It is no surprise, then, to hear that when the Huddersfield job came up, Wagner sought Klopp’s advice.

The thing was, his friend had something to get off his chest as well.

“In this business, with your best friends you have to keep a few secrets,” Klopp revealed on Tuesday.

“He didn’t know I went to the USA to speak with our owners and I actually had no idea that he was in talks with another English club.

“When I came back from the USA, I opened the door and first of all I saw his car sitting in front of my house and I thought ‘What’s he doing here?’ When I came in my wife said ‘Let him tell first!’

“I said ‘You first.’ He said he had spoken to a Championship club in England. I said ‘Interesting, which club?’ He said Huddersfield and I thought ‘OK, close to Liverpool.’

“I said I had come back from talks with Liverpool! So that’s how it was and a completely strange situation for both of us.”

Like Klopp, Wagner has had to adapt quickly to English football’s, its intensity and language - “gegenpressing”, for example, has been renamed “reactive pressure”. It hasn’t been plain sailing, on or off the field.

Huddersfield finished 19th last season, just a couple of points above the relegation places. They have neither the biggest budget nor the deepest squad in the Championship; they are having to “find solutions”, as Klopp may say.

Liverpool have helped, allowing Danny Ward to move to Yorkshire on a season-long loan deal. The Wales goalkeeper will not feature in Wednesday’s game, as he still on a post-European Championships break.

For Klopp and Wagner it will be the first time they have come face to face as managers. Klopp is looking forward to it.

“We are pretty close to each other and it’s good,” he says.

“He’s a really good friend but more importantly for Huddersfield he is a brilliant manager. So it’s a win-win situation!”