Rafa Nadal does not believe it. He never has and he never will. And that, he maintains, is the secret of his success, the reason why, embarking on the 2014 season, he goes into the Australian Open next week as number one in the world tennis rankings, why he has won a daunting 13 grand slam titles and why he is one of the most popular and successful sportsmen alive.

What does he not believe? On a midnight boat ride along the river Thames, he explains. “The acclaim, the success. That I am as good as people seem to think, or as the numbers say I am.” Why doesn’t he believe it? “Because the moment I did it would be all over. I’d be finished.” Finished? “Yes. Finished.”

Humility in Nadal is not an affectation, it is a strategy. But it is one that comes naturally to the 27-year-old Spaniard, though to look at him he hardly seems the diffident, retiring type. Taller in person than he appears on TV, he has a large, lithe, feline presence and while you sense when you are among a group of rival tennis pros that they shrink when he enters the room, he does not bear himself like an alpha male, nor does he see himself as such. To do so, he believes, would kill his appetite for victory.

We explore the theme sitting side by side on a crowded passenger boat travelling from the O2 arena, a giant white tepee in London’s southeastern extremity, to his hotel on the Thames, facing the Houses of Parliament. Even though he has just lost the ATP World Tour Finals to Novak Djokovic, the successor as his arch-rival to the great Roger Federer, he shows no signs either of being deflated or physically drained. He had had a bad day with his serve, he told journalists after the game, but otherwise he had done the best he could. What he hates is losing when he feels that he has given less than his best. After Federer defeated him in the 2007 Wimbledon final, he slumped in despair on the floor of the dressing-room shower for half an hour. He had let himself down, he had lacked concentration, he had played the critical points thoughtlessly. But this time, no recriminations or complaints. Djokovic had won fair and square. Just as Nadal had done when, with rather more at stake, he defeated Djokovic to win the French Open at Roland Garros last June and the US Open in New York in September.

Nadal after winning the third set in his match against Juan Martin del Potro of Argentina, Wimbledon, June 2011

That was his 13th grand slam win, four short of the record held by Federer. Nadal won his first major, the French Open, in 2005, aged 19. He has since added seven more French wins, two Wimbledons, two US Opens and one Australian Open. But his is not the rags-to-riches story people tend to associate with sporting superstars. Born into a comfortably off family in the unprepossessing Mallorcan town of Manacor, inland and rarely visited by the nine million tourists who flock to the Balearic island every year, he began playing tennis at the age of three with his uncle Toni Nadal, a coach at a local club. Furiously competitive from the start, loathing to lose even at family games of cards, he was an athletic prodigy who when he was a little older would face a difficult choice between a professional career in tennis or football, a game at which he excelled. Another uncle, Miguel Ángel Nadal, Toni’s brother, played for Barcelona and Spain.

The question was settled when he made it to the final of the Spanish national under-14 championship aged 12. Nadal wasn’t even a teenager but he already had a career. He won his first game in the top tier of world tennis, the ATP, when he was 15; he beat Federer for the first time when he was 17; at 18 he won the decisive singles match that gave the Davis Cup, the tennis world cup, to Spain. The expectations he generated were massive but, despite being repeatedly laid low by injuries, despite fearing when he was 19 that a congenital bone fissure in his foot would end his professional life, he wildly exceeded them in following years. Yet no year, he tells me on the riverboat, has given him greater satisfaction than this past one. He started the 2013 season one month late, having slipped down to number four in the world rankings after a seven-month lay-off forced by a recurring injury to his left knee. During much of that time he had not been sure he would ever play again. By the end of 2013 he was back at number one.

Training in Mallorca, aged 15

That is why he is so cheerful despite the loss to Djokovic. “I’m 100 per cent sure this has been my best year,” he says, speaking in Spanish. “Not only could I not compete during those seven months, I couldn’t train on the court. To come back from that and then to have played in 14 finals, winning 10, is, well, una barbaridad.” The literal translation would be “a barbarity”; what he means, though he says so himself, is that what he has achieved has been pretty damn remarkable.

Nadal is not falsely modest, then. He is self-effacing only up to a point, which is a relief because his relentless niceness, that extreme courtesy he always extends to his opponents after battering them on court, might otherwise feel like some sort of artful marketing ploy. He is human enough to be able to celebrate his past achievements. The doubts only come when he looks to his next game.

…

Animated, smiling broadly as we sail past the tall bright lights of Canary Wharf to our right, then under Tower Bridge, then past Shakespeare’s Globe theatre to our left, he expounds on his philosophy of humility as the recipe for success. “Look,” he says, “it’s also true, and you will agree with me, that there are guys right at the very top who are super arrogant and are super sure of themselves. There isn’t only one way, clearly. But my way is this: when you believe that things are going to go your way just because you are who you are and because you are very good, that’s when you start to get careless, when you stop working hard, when you stop fighting for every point. You lose your intensity and while things may go well with you for a while, because you’ve built up a positive momentum, it won’t last. You drop your guard when you stop having doubts about yourself. I’ve always had doubts. I’ve never believed that I am very good, nor do I believe it now.”

I know Rafa. We travelled far and wide when we worked on his memoirs together three years ago. He is the most grounded and understated of superstars. But this feels like a bit too much. “Not very good? You cannot be serious,” I say.

“No, no, no,” he replies, chuckling. “I am serious. It’s evident that my numbers say I am good, sure. I know I am number one because the numbers say I am. But I don’t see myself that way.” A case of imposter syndrome, then? I ask. Maybe in his heart he feels he does not deserve to be where is?

“Yes. Something like that. I mean, for example, when I go out on court I don’t believe myself capable of . . . How do I explain it? . . . I see it as very difficult to win the US Open, to win Roland Garros [the French Open]. I feel it’s very, very difficult to win these things. Do you understand me, or not?” He breathes in deep, lets out a long “Oofff!”, as if reliving his sensations as he approaches a grand slam final.

“Take Tiger Woods,” continues Nadal, who is a mad keen golfer. “I look at him playing a shot and I think, he is going to nail it, because this guy is very good and he knows he is very good.

Roger Federer (Switzerland) at the Australian Open, January 2013. Age: 32. Grand slams: 17

I look at Federer and Djokovic – the same. But me, even though rationally I know they probably regard me in the same light, I really do have my doubts. I’m about to start a new year, a new season and I have no idea whether I am going to win anything at all. I know that when I arrive in Australia I’m going to feel that it’s going to be very tough for me, very hard to win there. Once the time comes and I am out on court it’s different. I am prepared to compete well and I know that in the moments of maximum pressure I’ll respond the right way. But never do I think I am going to go out and win because I am better than the other player. I’ve never felt that.”

He means it. Though he comes across as more assertive now than when we last chatted at length, in 2011, more comfortable in his own skin, he really does believe that he is not all that he is cracked up to be. It’s served him well. Ever since he first hit some balls across a net with Toni, the refusal to believe he is better than the other players has led to him winning nearly nine matches out of every 10. It is the reason why, by all accounts, he trains more intensively, both on court and in the gym, than any other player in the professional circuit. I’ve spent long hours, often sitting next to his father, watching him play practice sets in empty stadiums. Each time his father would warn me to keep as quiet as if it were a Wimbledon final, for Rafa would get cross if he caught us nattering. Tennis is work for Rafa and work is serious business.

I don’t believe the acclaim, the success. Because the moment I did it would be all over. I’d be finished

An absence of complacency might be in his nature but it also has a lot to do with how he was brought up. When Nadal dedicates a victory in a post-match speech to his family, thanks them as he invariably does for the impact they have had on his triumphs, he speaks from the heart. If there is one thing of which he is convinced, it is that he would not be where he is were it not for the influence of his remarkably close, traditionally patriarchal family clan. For a while, and until quite recently, the entire Nadal family (half of whom, incidentally, are on the Thames riverboat with us) lived in one block of flats overlooking the church in Manacor’s central square. Grandfather Nadal, a retired musician also called Rafael, occupied the ground floor with his wife. Above him were his oldest son Sebastian, a successful businessman, and Rafa’s father, his wife Ana María Parera, who comes from a furniture manufacturing family, Rafa himself and his younger sister Maribel. Also in the building, which they wholly owned, were grandpa Rafael’s three other sons and one daughter, all married, all with their spouses and offspring.

From left: Nadal wins a regional tournament for under-12s, aged nine; with uncle Toni, his coach, Paris 2005

Uncle Toni is the one most often credited with sculpting the modest court assassin, though Nadal always stresses that his parents and sister deserve at least as much thanks. He attributes his success as much to the person he was raised to be at home as to his training as a sportsman. But it was Toni who drummed into him the idea that every opponent had to be treated with anxious respect, that he should never rest on his laurels. On the night after the young Rafa and his family celebrated his winning the Spanish under-12 championship, Toni poured cold water on the proceedings by reading out a list of the previous 25 winners of that competition. How many of them have you heard of? he asked Rafa. How many went on to achieve anything in tennis? Rafa acknowledged he had only heard of two or three. There you are, then, said Toni, with a nod of satisfaction.

On another occasion, soon after, Nadal was flying back from South Africa, where he had just won a world tennis junior competition. His paternal aunt prepared a party at her home, complete with bunting and balloons. Toni found out, went around to her house and tore the bunting down. There would be no party. That evening, he informed Rafa, who had just arrived, that he’d expect him for training the next morning at eight.

…

From left: playing football in Mallorca, aged eight; at 13, with father Sebastian and sister Maribel at her communion

His mother and father and his sister, with whom he speaks every day no matter where in the world he is, looked askance at times at Toni’s somewhat brutish regime (if as a young boy Nadal forgot to bring a bottle of water along to a match on a hot day, no water was given to him). But they were always careful during his roaring teens to ensure fame never went to his head, mercilessly teasing him at the merest suggestion that it might. When he succumbed to the temptation of buying himself an Aston Martin after winning Wimbledon in 2008 aged 22, it was over the objections of his father and sister, who to this day refuses to be seen in public anywhere near it. Mallorcans in general, and the Nadals in particular, are a self-effacing lot.

I have learnt to distinguish clearly between Rafa and Rafael. Rafa is the famous tennis player; Rafael is the real one

Women might have been another temptation, yet Nadal has remained true to his girl-next-door girlfriend, María Francisca Perelló of Manacor, since they started going out nine years ago. An even deeper measure of how rooted he is is that the idea of doing as other sports millionaires do and moving to Monaco, say, to avoid paying the Spanish taxman has never crossed his mind. Not only does he remain, and will almost certainly remain all his life, in Mallorca, he lives with his parents in a wing of a home he bought for them on the sea, 10 minutes’ drive from where he was born. Fame and wealth have not devoured the man.

Nadal’s girlfriend, María Francisca Perelló (in blue scarf ) with family members and friends at the Monte Carlo Rolex Masters, April 2013

“If there is one thing I have learnt,” Nadal told me once, “it is to distinguish clearly between Rafa and Rafael. Rafa is the famous tennis player; Rafael, the name they call me at home, is the real one, the one who could have ended up doing something else altogether with his life and it would have made no difference.”

The duality in his person is strikingly apparent also in the difference between the gladiatorial tennis player one sees grunting, battling epically for every point on court and the gentle, contemplative soul off it. The odd thing about Nadal, and the appealing thing for a large part of the tennis public, is that he is a champion a large part of whose charisma derives from playing the part of the underdog. It doesn’t matter whether he is playing against the regal Federer, the flamboyant Djokovic or a player who is at 100 in the rankings, he always comes across as the fighting street-kid, scampering to salvage victory from the jaws of defeat.

But his toughest battles have been against the enemy within, the injuries that have made many tennis observers wonder time and again whether his career would be curtailed. He, too, has wondered and the fact that he is still there, riding high, is why he derives such delight from his performances during the past year.

Rafael Nadal (Spain) at the US Open, Sept 2013. Age: 27. Grand slams: 13

“People say I am playing more aggressively, higher up the court, that I am conserving my energies better, playing more astutely than before,” says Nadal, as the riverboat approaches the Millennium Bridge. “I don’t like that. It’s not the point. My success has been a mental success. I wanted to prove to myself that I could compete at the highest level again. That’s the challenge that’s driven me this past year. It’s a question of pride. It’s been a personal battle. Because, let’s be clear, my knee still hurts me today. After winning at Indian Wells [in California] early in the year I was limping for 48 hours. I have played every game this year with anti-inflammatories, without exception. I have no choice if I am to remain the player I know I can be in the face of my body’s limitations. And that constant battle with myself is what motivates me as much as anything else.”

Andy Murray (Great Britain) at Wimbledon, June 2013. Age: 26. Grand slams: 2

Few players who compete against him would be surprised. The consensus in the tennis world is that it is Nadal’s mental strength, seen especially vividly in the exultant manner he rises to the occasion in the big points, that sets him apart. He has an inimitably powerful, topspin forehand but otherwise his serve is not all that special, nor is his backhand, nor his volley at the net. In the purely technical aspects of the game, Federer and Djokovic are, as Nadal is the first to acknowledge, more complete. Yet, as often as not, he outstays them, and outruns them, making up with focus and desire what he may lack in natural artistry.

Nadal points out to me that one should not forget Britain’s Andy Murray, who has suffered his own injury calamities this year, in the list of current greats. Between the four of them they have won 37 of the last 40 grand slams. “I feel fortunate to have been able to form part of this foursome, a very distinguished group in the history of our sport. It’s an amazing rivalry and it’s been great for tennis. Sports are nourished by stars and stars become stars when they’ve been there many years, when traditions are built up, big rivalries, games that are of interest to people beyond the sport itself.”

I feel fortunate to have been able to form part of this foursome, a very distinguished group in the history of our sport

When he reaches the point that he no longer forms part of the sport’s pack leaders might he retire? Could he tolerate dwindling into the sort of second-tier player who rarely makes it beyond the quarter-finals? “It depends if I am happy or not,” he swiftly replies. “If I get to the point where I know I cannot compete at the same level as before but I accept the situation and find I am still enjoying myself, then great. If that realisation becomes too heavy a weight to bear that will be another thing. I don’t know right now, nor do I know how I will be physically in the years ahead. If every day becomes a calvary, if the pain becomes too great, then that will decide it for me. Now I cannot tell.”

Novak Djokovic (Serbia) at the China Open, October 2013. Age: 26. Grand slams: 6

What he does know now is that he is looking forward to competing in Australia next week with as much enthusiasm as the first time he played there 10 years ago. “It’ll be complicated, it will be tough. Winning there right now feels like an impossible challenge,” he says. “But I look forward to it with enormous excitement. It’s one of my favourite tournaments and yet it is the one where I have been most unlucky, either because injuries have kept me away or because I lost a final against Djokovic in 2012 that either of us could have won after six hours of play.”

Djokovic often looks a more commanding player than Nadal yet has won seven fewer grand slams. It is unlikely Djokovic will catch Nadal but it is not out of the question that Nadal will equal or beat Federer’s so far unique haul of 17. A last question, as it is nearly one in the morning and our boat is pulling in to moor alongside the London Eye. Is it important to him to achieve that feat, to beat the grand slam record?

“I don’t think about it,” he says, smiling a little shyly. “But you ask if it is important. Of course it’s important for me! But I don’t think about it today. Today I am thinking only of getting ready for Australia. Never in my life did I set myself the target of winning 13 grand slams. But I do have them now. Let’s see what happens in the future. I never imagined myself breaking any records but, well, look: here we are. Let’s see how far we go.”

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June 3 1986: Nadal is born in Manacor, Mallorca, Spain

1989: Starts playing tennis aged three with his uncle Toni, who remains his trainer

2001: Turns pro, aged 15

2004: Nadal’s victory over Andy Roddick helps Spain beat the US in the Davis Cup final

2005: Wins the French Open, aged 19, on his first attempt

2006: Wins the French Open again, becoming the first player to defeat Federer in a grand slam final

2008: Ascends to world No 1 and wins gold at the Beijing Olympics

2010: Becomes the second man after Andre Agassi to complete a career golden slam (four grand slams plus an Olympic gold)

2013: Wins the French Open, becoming the only male player to win it eight times

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The Australian Open runs from January 13-26; www.ausopen.com. John Carlin is co-author of ‘’ Rafa: My Story’ (Sphere) with Rafael Nadal.

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* This article was amended on January 13 to reflect the fact that Rafa Nadal was the first man to win eight French grand slams but not in a row, as previously stated, and that Novak Djokovic is younger than Nadal not older