Dusan Vlahovic was an academy player still two months shy of his 16th birthday when the Partizan Belgrade coaching staff decided he was ready for a trial run with the senior squad.
The plan was for the young striker to take part in first-team training for a period of a few weeks, gaining invaluable exposure to men's football before returning to Partizan's academy to continue his development with the U16s. But the plan soon changed.
"After one or two training sessions, our head coach, Ivan Tomic, said, 'This guy is a monster, this is a player who will be worth 100 million euros one day,' because he showed unbelievable character," Zarko Lazetic, Partizan's assistant manager at the time, tells Sky Sports.
"I remember we played a game of 10 against 10. We put him against our vice-captain, Darko Brasanac, who now plays in La Liga, and he pushed him around like he was already a first-team player. He showed unbelievable things for a player of 15."
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Vlahovic was promoted to the senior squad permanently and handed his competitive debut just a few weeks later, becoming Partizan's youngest debutant at 16 years and 24 days old.
Soon after that, in April 2016, he became their youngest scorer.
"We were losing 2-0 to Radnik Surdulica," recalls Lazetic. "Dusan came on at half-time, scored the first goal and we won 3-2." Vlahovic then scored again in the Serbian Cup final, helping Partizan beat rivals Javor 2-0. "By then, everybody knew how everything would be in his career," Lazetic chuckles down the phone.
Six years on, Vlahovic is causing a stir again.
Only this time on a much bigger scale.
The extraordinary potential first seen in him back in Serbia is being realised at Fiorentina, the club he joined from Partizan in 2018 following a recommendation by their former striker Valeri Bojinov.
Across 2021, Vlahovic plundered 33 league goals in 43 games, a total second only to Bayern Munich's Robert Lewandowski in Europe's major leagues.
With it, he equalled a Serie A record for goals scored in a calendar year set by Cristiano Ronaldo. Still only 21, his scoring feats have placed him in esteemed company. They have earned him a long list of suitors too. Arsenal are trying to sign him this month.
Arsenal's interest is no great surprise given Vlahovic offers qualities they sorely lack up front. As well as being a lethal finisher, the Serbia international is an imposing figure at 6ft 3ins. He is powerful but also quick; aggressive in style but also capable of a light touch.
His aerial ability is sure to appeal to Mikel Arteta, who is seeking to maximise Arsenal's threat from set-pieces having added set-piece specialist Nicolas Jover to his coaching staff during the summer. But Vlahovic would bring plenty more besides.
Indeed, while the majority of his goals for Fiorentina have come from relatively close range in the centre of the opposition box, where he times his runs intelligently and finds space to dispatch crosses and cut-backs with impressive precision, his threat is not limited.
His ball-striking ability makes him dangerous from long distance too. He is often entrusted with free-kicks at Fiorentina and he is also deadly from the penalty spot. He has scored 12 of the 13 penalties he has taken in Serie A. Arsenal, by contrast, have missed three of their last five.
Vlahovic's athleticism means he is well-suited to playing on the counter-attack as well as fulfilling the role of a target man - his actions by zone map shows his tendency to drop deep in search of possession - but those who know him best consider his mental qualities to be most crucial.
"He has a very good left foot, he can score goals from free-kicks, from penalties, but the reason everything is possible for him is because he has a very, very strong character," says Lazetic.
"Character is the most important thing for any young player because it is what gives them the opportunity to show their quality. A lot of players have the talent, but because of character, they don't always show it. Dusan had a good personality and a very good work ethic. Every day he wanted to get better and better in all aspects of football - physically, tactically, technically. That was the difference."
Lazetic, now head coach of Partizan's Serbian Superliga rivals TSC Backa Topola, laughs at the memory of taking calls from a teenaged Vlahovic on what were supposed to be days off.
"When I was assistant and he was 16, there were times when we won games but he didn't play. The next day was always a day off, but he would call me and say, 'Please, coach, can we do a training session?' We did a lot of that, shooting practice or something else."
Vlahovic's age and profile have prompted comparisons with Borussia Dortmund's Erling Haaland in recent months but he spent his younger years idolising Zlatan Ibrahimovic.
Lazetic remembers the youngster sitting in the Partizan dressing room before games poring over clips and highlights of the Swede in action. "That's his obsession," says Lazetic, "to be like Zlatan."
Vlahovic is outscoring his idol in Serie A now - "it's an example to young players that dreams can come true," says Lazetic - but it might not have happened without the single-mindedness and determination that has been with him from the start.
Indeed, it was only after Cesare Prandelli's appointment as manager in November 2020 that it all came together for him at Fiorentina. "I owe him everything," said Vlahovic last year. Before that, his opportunities had been limited; his scoring record modest.
"For sure, he has reached this level very quickly, because one year ago he liked like a normal player for Fiorentina with not a lot of goals," says Lazetic. "But everybody who has worked with him knew that one day he would reach this level."
Prandelli has since been replaced in the dugout at Fiorentina but Vlahovic's importance to the side has grown.
The Serb has become a leader, shouldering huge responsibility for someone so young. Last season, his goals saved Fiorentina from relegation. This time around, they are pushing them towards Europe.
Expectations have risen but he has handled the increased pressure to perform with ease. Remarkably, his goals are worth 30 points to Fiorentina since the start of last season. No player in Serie A has won more points for his side in the same timeframe.
Vlahovic's personality and mental fortitude are just another part of the attraction for Arsenal. Arteta is overseeing what he has described as a cultural overhaul of the squad and has spoken of character as a key factor in the club's recent recruitment choices.
The boy from Belgrade has plenty. It has helped him become one of the deadliest strikers in Europe already. But at 21, and with suitors circling, this is still only the start. "Nobody knows where his limit is," adds Lazetic. "He is looking like he can touch the sky."