It's a measure of how highly Manchester City and Liverpool regarded Ilkay Gundogan that he was still laid up after knee surgery when they fought to sign him in the summer of 2016.

Pep Guardiola wanted to make Gundogan his first signing at the Etihad. Jurgen Klopp wanted to be reunited with his former Borussia Dortmund midfielder. Guardiola won.

Technically adept on the ball and tenacious in winning it back, Gundogan is an ideal fit for the high-intensity style of two coaches who meet head on at Anfield on Sunday.

Ilkay Gundogan admits it was tough to turn down Jurgen Klopp after joining Manchester City

Gundogan shared a close relationship with Klopp during their time at Borussia Dortmund

The 27-year-old Germany international will address the Liverpool boss by his nickname 'Kloppo'. They remain friends even though Gundogan turned down his old manager and joined City for £20million instead.

'I spoke with Jurgen about different things,' Gundogan says. 'He always liked me as a player and I would be a liar if I said he didn't try. But when I had the opportunity to join City and work with Pep, it was quite clear that I wanted to come here.

'When I was a little bit down because of my injury, he was quite sure he was still going to try to buy me. That showed me it could never be the wrong decision to join this club. They are both great managers, great characters and very ambitious. I've been lucky.'

Gundogan won the Bundesliga title at Dortmund and also reached the Champions League final in 2013, scoring a penalty in the 2-1 defeat by rivals Bayern Munich at Wembley.

But the chance to work with Pep Guardiola at Manchester City was too good to turn down

The Man City midfielder is happy to be discussing football again rather than his injury misery

In a biography on Klopp published in September, he revealed that his old boss was not averse to allowing the occasional team drinking session.

He also recalls white-water rafting trips on pre-season camp in Austria. 'A few times, to build the spirit.'

But above all, Gundogan knows just how difficult it will be to subdue a Klopp team in full cry at Anfield.

It remains the one Premier League ground where Man City have failed to win since the Abu Dhabi takeover in 2008. You have to go back almost 15 years for their last victory there.

If Liverpool cannot derail City's unbeaten and seemingly unstoppable title charge, who can?

Gundogan will reunite with Klopp when City travel to Anfield for a league clash on Sunday

The Germany international opened up to Sportsmail at City's state-of-the-art training base

'It could be a big moment for us,' says Gundogan. 'Anfield is one of the hardest stages in the world and on good days Liverpool are able to beat any team in the world.

'It's more exciting for everyone when you have two top teams who try to attack and try to create chances.

'I can imagine it's not attractive for the spectators when we play teams with 10 players around their own box, just defending and hoping for a set-piece or throw-in, anything.

'We need to be ready to show the same spirit as we did away at Chelsea and Manchester United. We have beaten all the big teams until now and we want to continue on Sunday.'

It has raised talk of an unprecedented Quadruple. The runaway Premier League leaders have one foot in the Carabao Cup final after beating Bristol City earlier this week, and will be strong contenders in the Champions League and FA Cup as well. So can they win all four?

He will be part of Guardiola's team looking to maintain City's unbeaten record in the division

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'It's difficult,' says Gundogan, breaking into a smile.

'If I said yes now I would disagree with the manager, and that wouldn't be so smart! We have a lot of games to go, and a lot of hurdles to take, but everything is possible.'

What about emulating Arsenal's Invincibles?

'We are in a comfortable situation and enjoy being on top.

'Having that gap also gives us confidence. This season is special as we are winning and winning and winning. We will try to go all the way. Maybe we will lose one day and maybe we won't.

'We always feel safe. In games where we don't score many goals, we still feel quite sure we won't concede much. And in games where we concede one or two, we feel we can score a lot more.

'At this point, it just fits automatically. It's just there. We don't think about it much. On the field, everyone knows exactly what we need to do. Sometimes it's strange.

'I have never felt at any time this season there is a safe first XI, just players who play more and some who play less.

'There is even more potential in this team. We didn't play badly last season but it was more up and down. This season, we are just on a wave and it's very hard to get us off this wave.'

The softly-spoken midfielder is only just starting to show his best after joining City in 2016

Softly spoken and unfailingly polite, Gundogan comes from a close family. Among others, it includes older brother Ilker, cousin Ilkan, father Irfan and his uncle and agent Ilhan.

'We have quite similar names in the family,' says Gundogan, grinning again. 'Especially the men's names. I don't know whose idea that was.'

His grandfather Ismail emigrated from Balikesir in the west of Turkey and settled in Gelsenkirchen, where he worked down the coal mines.

Ilkay was the third member of the family to be born in Germany. His father worked as a delivery driver for the Stauder brewery in Essen and his mother Ayten was a cook at a health club. Their two boys started playing football at SV Gelsenkirchen-Hessler 06.

His tenacious style and creative talents made him the perfect fit for Guardiola's system at City

'The idea of my parents was to keep us away from the streets,' says Gundogan, who came through the youth ranks at Vfl Bochum before making his name at FC Nuremberg. 'Gelsenkirchen is not a rich city. The crime is above average so they always tried to keep us away from bad things, and I think they were successful.

'My parents wanted me to have a good education, so I finished school, and at the same time I was able to play football. I grew up in a very multi-cultural society.

'Still today, my best friends are German from Morocco, from Tunisia, from Poland. Very mixed. I always saw it as a nice thing, seeing different cultures. It felt like a new experience.'

It is one of the reasons Gundogan, like Guardiola, has chosen to live in a city-centre apartment with his cousin Ilkan rather than the Cheshire countryside. The millionaire lifestyle is not for him.

Much of his time at the Etihad has been spent on the sidelines with a serious knee injury

'Living in a big house on my own just with my cousin, I think we would have felt a little bit lonely, especially when we were outside,' says Gundogan.

'I heard that sometimes you can drive 40 minutes to training and other times one hour. That wasn't an option for me. Because I was coming to a new country and city, I wanted to move into town and see how Manchester is; go out for a coffee or shopping and have everything close to me.'

'My knee doesn't feel like that of a 20-year-old, I have to take care of it more now'

Sometimes it felt like a prison in the long months of rehabilitation after Gundogan ruptured the cruciate ligament in his right knee in 2016. A player who had a full year out of the game due to a back injury suffered in 2013 — then dislocated his right kneecap shortly before moving to City — was hurt in a game against Watford.

He went to Barcelona to have surgery with Dr Ramon Cugat at the Quiron Hospital and spent a month living in the city with his fitness coach Arthur Jankowski and a private chef.

Now, the Germany international is finally starting to show why Guardiola wanted him so badly

BACK TO HIS BEST 2 - Ilkay Gundogan was Manchester City's most creative player against Burnley in the FA Cup last weekend. He assisted two goals and created another two chances in City's 4-1 victory. Advertisement

Gundogan returned to Manchester and began a gruelling rehab programme for six days of the week at the City Football Academy. Three bouts of fever set him back and he had his tonsils removed.

He finally returned to the City squad in September after nine months out, as an unused substitute against Liverpool at the Etihad — a thumping 5-0 triumph after Klopp's side had Sadio Mane sent off.

Now he is happy to be discussing football again rather than his injury misery, talking about contributing to the team.

Gundogan provided two assists for Sergio Aguero against Burnley last weekend and played in the cup win over Bristol City four days later without any ill effects.

Gundogan provided two assists for Sergio Aguero in the FA Cup victory over Burnley

'I can see the finish line to my old form,' he says. 'That feels more important to me than assisting or scoring. Obviously my knee doesn't feel like that of a 20-year-old who is at the beginning of his professional career.

'I need to do things to take care of it. Having treatment every day, and my routines before and after games.

'I remember when I was at Dortmund, I would go outside and kick the ball into the goal straight away, taking free-kicks or something.

'When I see the young players doing that, I think, 'Oh my God, no!' I won't do it now, and maybe I couldn't even do that. I'm quite sure that I will never be the same player again that I was. But that doesn't mean I will be weaker or not of the same quality. Just a different player.

'Honestly, I don't even regret it. I just think that these kind of things happen in football and life.

'I try to see it as an experience that I accept and take with me.'