Andreas Engelmark still remembers the first time he coached Dejan Kulusevski in an academy game for Stockholm club IF Brommapojkarna.
Engelmark, who was then a youth coach at Bromma, was excited by the potential of the gangly forward, who loved to dribble and had "a big drive that was unique" for a 12-year-old.
But there was a problem.
"He didn't like to defend," says Engelmark, now Bromma's co-head coach.
"We lost the ball and he was just a few metres away from the player with the ball and could press them obviously. But he didn't do it and I said, 'well, what do you think you're going to do when we don't have the ball? You need to defend!'"
Twelve years later, Kulusevski is not just a hard-working player by professional standards, he is probably the hardest-working player in the Premier League – a pressing machine who has been transformed since moving from the wing to a central role for Ange Postecoglou's Tottenham.
I just have one life, just have one career ... when I'm done with this, I'm done. I want to have no regrets
Last season, no one in the top flight covered more ground in a single game than Kulusevski's 13.36 kilometres in Spurs' 2-1 home win over Everton, while after 12 games of this term, he has run further per match and won possession in the final third more times than any other player.
In fact, Kulusevski is close to overhauling Newcastle's Bruno Guimaraes for the most ground covered in the Premier League this season, despite sitting out 79 minutes of Spurs' opening game at Leicester.
The Swede is no plodder, either, and only nine players in the league have clocked up more sprints than Kulusevski.
The 24-year-old is becoming English football's marathon man, setting new standards with his remarkable stamina – and he knows it.
"I have something that no other player has," Kulusevski said after Spurs' landmark 4-0 win over Manchester City on Saturday. "So you can’t compare me to any other player.
“With my engine, with my heart, I don’t get tired. I can’t let anybody outwork me.”
It is quite the turnaround for the young player who hated to track back, and Engelmark says Kulusevski's development was a "a process" over their three years together at Bromma, with the penny finally dropping shortly before he moved to Italian club Atalanta, aged 15.
“He started working really hard towards the end of his time at the club,” Engelmark says. “In Italy he had to keep doing it, he learnt a lot in Italy in those three years."
Today, there is no great secret to Kulusevski's energy according to his colleagues at Spurs, just a blend of good genetics and hard work.
"I think he has no lungs! He can run all the time," says Spurs captain Heung-min Son told Standard Sport. "What I like about him is people see what he's doing with the ball but behind the scenes he's working so hard.
"I just want to mention that, for the people not seeing that attitude and effort."
With my engine, with my heart, I don’t get tired. I can’t let anybody outwork me
Kulusevski has made changes to his lifestyle as he has matured, focussing more on his eating, drinking and sleeping to maximise his physical condition, and trying to limit time spent doom-scrolling on his phone.
“I just have one life, just have one career and I just want to maximise it as much as possible because when I'm done with this, I'm done. I want to have no regrets,” he said after another big shift in Thursday’s 2-2 draw with Roma.
He has also spoken positively about the impact of the birth of his daughter, Leonie, in April, saying becoming a father gave him both fresh motivation and perspective.
For Postecoglou, who pushed hard to ensure Kulusevski’s loan from Juventus was made permanent last season, his engine is down to a number of factors.
“It’s not a fluke. It’s a lot of hard work,” Postecoglou says. “There’s some science there, some genetics or DNA. He just has a real capacity to be able to run at a certain level consistently. His recovery rates are really elite in that he can go and rest quickly and go again.
“But there’s also that mental strength to do it. A lot of players have the capacity but maybe reach the limit where they naturally get tired and don’t go again until they need to. But Deki will keep going and going. It’s a big part of his football, it’s a big strength of his.
“I’m not a biomechanist, but his running efficiency is really good. He doesn’t seem to use too much energy. Or that’s how it looks to me. That could be a load of crap, I don’t know!”
Perhaps most importantly, the Spurs head coach says Kulusevski has a singular focus on self-improvement that often separates great players from good ones.
He studies other players, occasionally poring over footage of someone like Kevin De Bruyne, and on a typical day, he is usually one of the last players to leave Tottenham’s training ground, often mid-afternoon, a good hour after most of his team-mates.
“He's very ambitious about how good he can be and that's a great thing,” Postecoglou says. “I've just got a feeling with Deki that he's not going to settle for any level apart from being the absolute best and it's great for us.”
Kulusevski is so focussed on being the best, he even spends his summers training, joining Engelmark for boot camps during the off-season, when they work on fitness and "details" in his game. Last summer, the focus was mostly on refining Kulusevski's end product.
"It was a couple of his old friends and some former players of mine who were in the same team, including [Celta Vigo and Sweden defender] Carl Starfelt," says Engelmark.
"He wants to be fit when he goes back and works on some details. We usually do it during the summer when he's home."
Kulusevski's move to the middle this season (or, rather, his return to the middle, because Engelmark says he always played him centrally) has also been a factor, freeing him up to cover the length and breadth of the pitch as one of Postecoglou's twin No.8s.
"Me and Deki always had these conversations [about position],” Son said. “Even when Antonio Conte was here. He knew he was very strong and he wanted to play in the middle, which he suits perfectly.
“He gives you more legs and more energy, he's doing really well with his back to the goal."
It would be a disservice to Kulusevski to suggest he is all about stamina, though.
No player has created more chances in the Premier League than Kulusevski's 33 so far this season, making him both the most energetic player in the top flight and also the most creative.
Kulusevski can scarcely up his work rate but Postecoglou and Engelmark, probably the two coaches who know the Sweden international best, believe there is still enormous scope for improvement in his game.
“He can score more, he can make more assists,” says Engelmark. “We were discussing this when he trained in the summer.”
Postecoglou is likely to play a major part in continuing Kulusevski's development and the Australian believes he has a player who will continue to push himself to the limits.
“He's still relatively young and for his position, there's a lot of improvement in him,” says the Spurs head coach.
“He has this real laser focus about how good he can be and, for any manager, that’s what you want to see in players. Somebody who is not going to settle for whatever level he is at.”
Heung-min Son was speaking to Standard Sport at the launch of Tottenham Hotspur's new Official Furniture Manufacturing Partner, Sidiz.
