In her time, Anna Kournikova was probably the most famous tennis star in the world, women's or otherwise. As a 15-year-old, in 1996, she reached the quarterfinal of the US Open, losing to then world No 1 Steffi Graf. A year later, she was a semi-finalist at Wimbledon. She was the "next big thing" and the most sought after athlete in internet searches.

Kournikova retired in 2003 with a grand total of zero singles titles to her name. She never reached higher than eighth in the world rankings. And she left many casual fans wondering if her fame and hype weren't fuelled more by her looks and mega-sponsorship contracts than her ability.

Freddy Adu looks nothing like Kournikova. Yet he too could find himself in the "Kournikova trap": a vicious cycle of unrealistic expectations and enduring on-field disappointment fuelled by the needs of sponsors and marketing men.

Right now, Adu is probably the most famous US footballer in the world.

Still just 17, he has been playing professionally in Major League Soccer for three seasons, since becoming, at age 14, the youngest professional in the history of US team sports and, at the same time, the highest paid player in MLS. He is also his country's youngest international, winning his first cap at the age of 16. He has a $1 million a year endorsement deal with Nike and is a fixture in their footall adverts, in which he stars alongside Pele, with whom he bears a passing resemblance. He has even had a celebrity girlfriend, precocious R&B star JoJo.

And yet, having completed his two-week training stint ("Under no circumstances must it be called a trial" an MLS executive insisted earlier this month) at Manchester United, his future remains clouded. Right now, he's a gifted but entirely unpolished teenager and nothing more. And, worryingly, there is reason to believe he may never fulfill the path to superstardom laid out by Nike, the US Soccer Federation and his agents.

His early years are storybook stuff. Born in the coastal Ghanaian fishing town of Tema, local teenagers would invite him to join their games, realizing that, even as a six-year-old, he could hold his own against guys three times his age. At least, that's the version put out by his marketing guys.

When he was eight, his mother, Emelia, won the US green card lottery and emgirated to a suburb of Washington DC, where she worked two jobs to help support Freddy and his younger brother, Fro. Within a year, local coaches had spotted his talent. A family friend plucked him out of state school and paid for him to attend The Heights, a £7,000 a year private boys' academy where his talent could best be nurtured.

In the summer of 2000, he travelled with his district team to Italy to participate in an Under-14 youth tournament. Despite having just turned 11, he dominated the competition, which included youth sides from Inter Milan and Bayern Munich. He was named the tournament's best player. A few months later, Inter offered him and his family a package worth £400,000 to move to Italy and enroll in their academy. Adu's family turned them down and instead, a year later, accepted the US Soccer Federation's offer.

This basically involved Freddy joining the kind of training camp you might have seen in East Germany back in the day. Officially, it's known as the USSF Under-17 residency programme and it is meant for promising 15-to-17-year-olds to refine their skills. In reality, the camp - located in Bradenton, Florida, around the corner from Nick Bollettieri's famous tennis academy - is run directly by IMG and is essentially a footballer factory.

Adu, at 12, was the youngest boy there by several years but, from day one, was the centre of attention. Well aware that his success was not going to be simply a question of ability, they hired an army of tutors to help him academically (an outstanding student, he had already skipped a year and would eventually graduate high school at 15, two years ahead of schedule) and, tellingly, a full-time "mental conditioning" coach, Trevor Mowad. A big part of Mowad's job was to get the boy used to the media attention which was to follow. This involved extensive media training, including one exercise in which cameras followed him for most of the day. The tape was then reviewed privately to ensure that, even in his daily routine, he did not do or say anything which might reflect negatively on his image.

With this kind of preparation it's no surprise that, when he turned professional, he took the media attention in his stride. The sponsors felt they were on to a winner, going so far as to point out that Pele won his first World Cup in 1958, a few weeks after his 17th birthday and that Adu would have just turned 17 at Germany 2006.

Unrealistic? Probably. His development was not quite as rapid as was hoped. In his first two seasons he was in and out of the side. And, while he won his first cap at 16, it was also to be his only international appearance, as US coach Bruce Arena left him out of the World Cup squad. The season which just ended was his first as a fixture in the side. He started 19 regular season games, scoring two goals. For any other 17-year-old, being a bonafide pick at that age would be quite an achievement. For him - and, more crucially, public opinion - it felt anti-climactic. In fact, veteran US soccer pundits pointed out that he wasn't even the most hyped 17-year-old in MLS: that honour went to the New York Red Bulls' Josmer Altidore.

Officially, Adu views his time at United as a "getting-to-know-you" exercise and little more.

"While I'm hoping this all leads to a transfer offer eventually, my aim was to show flashes of the kind of player I can become," he says. "This was always going to be a test for me so I can gauge where I am by training with these guys."

To be fair, gauging where he is and what he can be is far from easy, largely because his game remains incomplete. He is an outstanding and creative dribbler who is also quick and imaginative. But he is also profoundly one-footed, a questionable finisher and his decision-making on the pitch is also suspect. While he can no doubt improve on the latter, the concern is that he won't be able to do much about his other shortcoming: size.

His height, depending where you look, is listed at either 5ft 7in or 5ft 8in, but many believe he is closer to 5ft 6in or even 5ft 5in. And, unlike other short players, he seems to lack the stocky base and low centre of gravity which would allow him to ride tackles and fend off opponents.

His natural position is in the hole as an attacking midfielder, but DC United boss Peter Nowak quickly realized that Adu could not play there: his distribution was not what it should be and he was simply too often outmuscled. That's why last season Nowak used hm as a wing-back in a 3-5-2, a decision which caused some controversy (or about as much controversy as soccer can cause in the US media). Adu was not exactly thrilled with his new role.

"At DC United I have not gotten much time at my natural position," he says. "I'd like to show what I can do in my real role and hopefully Sir Alex Ferguson will see that."

Such statements are probably more a sign of youthful exuberance than an inflated ego. Because the truth is that Adu knows all too well that he is probably not ready for the jump to Old Trafford or another European juggernaut.

Nowak caused waves when he suggested that Adu's development would be best served in Holland than at United. Some saw it as criticism, but, in fact, Adu is on the same wavelength.

"I want to be in a situation where I'm playing and I want to learn faster because I don't respond well to being brought along slowly," he says. "One of the big clubs could sign me and loan me to a Dutch or French or Belgian club for a season or two and then I could come back. MLS has been great for me, but to see a huge change in my abilities I have to go to Europe."

Perhaps that's what will help Adu avoid the Kournikova trap: his brain. Those who know him say he is exceptionally mature and cognizant of the pressure heaped upon him. And, rather than believing the hype, he truly wants to do what is best for his career. He already missed the boat on being the next Pele and leading the US to a World Cup triumph as a 17-year-old. But then, those are exactly the types of absurdly unrealistic expectations he doesn't need right now. What he does need is a club with quality coaching and the kind of competition that will enable him to fulfill his potential, whatever that may be (and it may well not be what we thought).

Oh, and some time away from the limelight wouldn't hurt either