How to describe and account for the brilliance of Lionel “Leo” Messi, the unassuming Argentine genius who is not only the world’s best footballer, but perhaps the greatest ever to play the global game? “With Leo,” says Thierry Henry, a former team-mate at FC Barcelona, “the best thing is not to talk about him. It is to watch him.”

Pep Guardiola, who, until he stepped down as coach at the end of May, was Messi’s mentor and protector at Barcelona, said something similar when asked to describe what made the little forward so great. “The superlatives ran out long ago. We have gone beyond words now. Don’t write about him, don’t try to describe him, watch him.”

So one recent evening I did just that, went to watch Messi in action against Málaga at his home stadium, the Nou Camp. It was Barça’s penultimate home game of what, compared with their recent triumphs under Guardiola, proved ultimately to be a disappointing 2011-12 season: they lost their league title to despised Castilian rivals Real Madrid and, as holders, were knocked out in the Champions League semi-finals by Chelsea, exponents of an ultra-defensive anti-football. For Messi, though, it was another long season of personal glory, in which he scored an astonishing 73 goals for Barça in all competitions (50 in La Liga), a new record for a European club season.

I’d seen him play – “play” being the operative word – twice before, against Arsenal in thrilling Champions League games at the Emirates Stadium in London. But it was a different experience altogether to be present among 88,000 Catalans in that vast open-air concrete bowl that is the Nou Camp during what were the last days of Guardiola’s reign as coach. What’s more, Messi, dressed in the famous maroon and blue striped shirt, scored a nonchalant hat-trick in a 4-1 win.

From my position high up in the stands, Messi seemed even smaller as he walked off at the end, the smallest man in a small side. He was delightedly holding the match ball, which in his hands became a basketball to be bounced, another object of play for the man who, as the Uruguayan novelist Eduardo Galeano puts it, “plays for the pleasure of playing”. Three days later, against Espanyol in Barça’s final home game of the season, Messi scored four, as one does.

Simon Kuper, in his book The Football Men, calls Messi the classic Argentine pibe, the man-boy who is blessed with the freedom and imagination of a very young child, dribbling without inhibition, always taking players on and running incessantly at them with the ball at his feet, as if he were still back on the street. Football as pure play. Football as an expression of childlike joy and simplicity.

So are you the classic pibe, I ask Messi, when we meet the day after the Málaga game in central Barcelona, where on this warm, breezy afternoon many of the roads are closed as students are protesting about a meeting of the European Central Bank.

“A pibe!” Messi looks up and smiles. “That’s what I’m trying to do,” he says, through a translator. He speaks Spanish in a quiet voice and with a strong Argentine accent. “Football is a game. I’m trying to have fun on the pitch, always, just to play. That’s why I do it. The day I stop having fun is the day I retire… I never want to lose that spark, that passion. Today, teams are playing more statically, more for the final score than producing good football. For them, it’s more important to win than to play well. We need more players with passion coming up for the good of football.”

Are Chelsea one of the static teams, I ask, reminding Messi, as if he needs it, of the penalty he missed against them during the second leg of the Champions League semi-final at the Nou Camp in April. (Chelsea drew the game 2-2, winning the tie 3-2 on aggregate.) “I felt terrible,” he says of the miss, and lowers his head. “Angry at myself, because I knew at that moment the whole tie was in my hands. But I can’t do anything now. It’s past. But it was a very tough moment for me and I still think about it.”

In person, Messi, who is fair skinned, is unremarkable except for his height – he is 5ft 6.5in, and yet seems somehow smaller, because he hunches a little and looks often at his feet, as if searching for the ball he feels should be there and with which he would be more comfortable, juggling it, rolling it back and forth.

He has dark, alert eyes, flat, unstyled hair and, on the afternoon we meet at the launch of a limited-edition Audemars Piguet Royal Oak Leo Messi watch, scruffy, gingery-brown stubble. He is wearing bulky white Dolce & Gabbana trainers and a white T-shirt, jeans that are more shredded than ripped, and a waist-length black leather jacket. “He could have made more of an effort,” one of the sponsors whispers to me, perplexed that he should have shown up for the event as if dressed for the terraces.

I liked the way he was dressed. There’s something attractively modest about Messi, both as a player and in his mannerisms: the slight shuffle, the polite awkwardness. “Humble”, “honoured”, “proud” are key words in his lexicon. “If there is a player who plays with zero adornments, it is Leo,” Guardiola has said. “I’ve never seen a stepover from him… He is the ultimate in effectiveness.”

Messi smiles easily in conversation and looks you in the eye. He has none of the ostentatious, rock-star strut of Cristiano Ronaldo, his only serious rival to the accolade of world’s best player and a master of adornment and the redundant stepover. Or the narcissism of David Beckham, who, out of boredom or self-love, or perhaps both, has turned his fine body into a tattooist’s fantasy canvas, a site of inky desecration.

Messi has no piercings and no tattoos. From what I hear and from talking to those who are close to him or have worked with him on business and charitable projects, he lives cleanly and quietly, in the Catalonian coastal town of Castelldefels surrounded by family and his Argentine girlfriend – his childhood sweetheart, Antonella Roccuzzo – all of whom he’s brought over from his home town of Rosario. At the end of 2011, he told Argentine magazine Caras, “I dream of being a father,” setting off rumours that his girlfriend was pregnant. In April, Roccuzzo apparently tweeted that she was expecting and rumours intensified when, after scoring his 23rd goal for Argentina against Ecuador in a World Cup 2014 qualifying match earlier this month, Messi stuck the match ball under his shirt.

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“Leo is not flashy,” says someone who works with him. “He has a Maserati and an Audi Q7, given to him by the club. He has a home cinema. He plays a lot of PlayStation. His life is quiet and it can be difficult. When he was in India and Bangladesh, playing an exhibition game, he couldn’t get out of the hotel because of all the fans waiting. It must be hard to live like that, with everything under scrutiny – what you can and can’t say, where you can go and when. He’s just a kid after all [he’s 24], who only wants to play football.”

Another business representative said to me: “Leo couldn’t look me in the eye when we met. It was as if he was embarrassed by the attention, ashamed to have such talent.”

I was told Messi earns £32 million a year, from Barcelona and from sponsorship and brand “ambassadorial” work. When I meet him he’s accompanied by his ever watchful agent, Pablo Negre, who inspected my digital recorder in a manner that suggested he expected it to explode, and his brother Rodrigo.

Everything he does in public is rigidly choreographed and his utterances are policed into blandness. My questions had been submitted weeks before to the agent; most had been rejected as “unsuitable”, including anything about the country of his birth – it’s said that Messi never plays as well for Argentina as he does for Barça and he will never be fully accepted as truly great by his compatriots until he consistently dominates international matches as Diego Maradona did. More than this, to be considered greater than Maradona, he will have to inspire Argentina to World Cup victory as Maradona did in 1986 in Mexico, when he played teams sometimes as if on his own.

It must be tough for Leo to live as he does, I say to his brother, everything controlled.

“It’s not so bad if you think of the alternative – that is, never to have been known at all, never to have had this happen.”

Still, and I put this directly to Messi, it must feel as if he lives in a kind of gilded prison, from which he escapes only to play football. “I try to live as normal a life as I can,” he says. “I try to go to the cinema, to shop in the centre, to go out to a restaurant, to take a stroll with my family – it’s the best thing for me to try to live like that. Fortunately, in the city of Barcelona people allow me to live as a normal person, like I am, and to enjoy my private life off the pitch. And my family helps so much. We don’t have many opportunities when the whole family can be together, nephews and cousins, aunts and uncles – these are very beautiful and important moments for me.”

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Born on June 24, 1987, in the industrial town of Rosario, in the Santa Fe province of Argentina, Lionel Messi arrived with his father, a steel worker, in Barcelona as a diminutive 13-year-old (he was diagnosed with growth hormone deficiency at the age of 11). He was only 4ft 7in, his legs were thin and spindly, and he required intensive hormone therapy to accelerate his growth, which no club in Argentina could afford, or at least was prepared to fund. But he performed brilliantly in a trial game and Barcelona were impressed enough to take him on.

Back home he was considered, for all his technical gifts and natural speed, to be simply too small to succeed as a professional footballer, but at Barcelona, with medical assistance, he grew taller and rose rapidly through the ranks. By the time he was in his late teens he was a first-team regular and already being spoken of as a talent for all the ages. His legs had strengthened, he had a low centre of gravity, preternatural balance and poise and he could run faster than any other player while keeping the ball under tight control at his feet. “Nobody was so wonderful [as Messi] at 19 years, neither Pelé nor Maradona,” says Karl-Heinz Rummenigge, the former Germany striker turned chief executive of Bayern Munich.

Now, Messi is considered to be Barça’s greatest player, greater even than Johan Cruyff, who did so much, first as a player, from 1973-78, and then as coach, from 1988-96, to create the modern Barça style of “tika-taka” football: the relentless, fast-paced and intricate passing game that has conquered the world. (The Spain national team, with its many Barça players, won the 2008 European Championship and 2010 World Cup playing a version of tika-taka.)

Messi could have played for Spain under residency rules but instead chose Argentina, yet he is accepted by Catalans as one of their own: a boy formed by and schooled at the La Masia, the stone farmhouse built in 1702 that is part of the Nou Camp complex. It is home to the club’s celebrated youth academy, which has produced players of extraordinary discipline and technical ability: Guardiola himself from an earlier generation; Xavi; Andres Iniesta; Gerard Piqué; Cesc Fabregas; Sergio Busquets; and so it goes on.

“Everything was new to me when I arrived there,” Messi says of his early months living in a dormitory at La Masia. “Everything seemed suddenly complicated. I had part of my family still in Argentina, I had my friends there. What helped was that I was living there with other young boys from Spain and from all over the world – being there all together made everything a lot easier for me.”

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How must it feel being Lionel Messi? How must it feel to wake each morning to the sound of the world’s applause, to know that you are not just exceptional, but the very best, and at football, the dominant cultural form of our time? What does this knowledge do to your motivation? How great is the desire to keep getting better, especially when you have as much money as you can ever need?

Such questions, Messi says, are “very complicated”. He prefers to play rather than to reflect and introspect. “I am playing for one of the best teams in history. I’m very grateful for everything life has given me so far, for everything I’ve been able to achieve, for the family I have, for the people who surround me. But I always believe better things will come. I want to grow and mature as a person. I still have so much to learn. I am the way I am at every moment. I am not playing a role. It makes it easier to be myself. I don’t have to watch what I do, I just do what I do. And remember: I am only 24 years old.”