Brazil's Kaka had just won the 2007 Fifa Player of the Year award but it was an awkward on-stage moment next to him that was to go viral and provide perhaps the first glimpse of a generational rivalry.

A youthful Lionel Messi and Cristiano Ronaldo had finished second and third respectively but Brazil legend Pele mistakenly handed the latter the trophy for second place at the Zurich Opera House.

Fifa president Sepp Blatter had to intervene to ask the duo to swap the trophies, with both looking suitably unimpressed.

For the next 10 years, Messi or Ronaldo won every Ballon d'Or and Fifa award going. In fact since 2007, 20 of the 29 gongs awarded for Europe's player of the year has gone to one of them.

With approaching 2,000 career goals between them, 85 trophies for club and country and countless individual awards and records, Messi and Ronaldo will forever be remembered as two of the most decorated players in history.

It is a rivalry that has defined the last two decades of football, transcending clubs, countries and competitions to reshape how the game has been played, consumed and debated across the world.

"Two players like them, competing at that level for so many years, fighting over the Ballon d'Or and scoring that many goals... I don't think we'll see it again," Argentina World Cup winner Angel di Maria, who played with both, says in BBC Sport's new documentary Rivals: Messi v Ronaldo.

Driven by a pursuit of trophies, records and a relentless desire to be the best, the pair – likely to be making their final World Cup appearances this summer - have pushed each other to new heights.

It is a rivalry that goes beyond the ultimate 'greatest of all time' debate they have fuelled.

"They both changed football," says Di Maria.

Ask anyone who is the greatest and agreement is hard to find.

"It has to be Ronaldo," ex-Manchester United team-mate Rio Ferdinand says.

"Messi is the best there has ever been," insists ex-Barcelona player and manager Xavi.

Ronaldo has had his say in the past, stating in 2012: "You cannot compare a Ferrari with a Porsche. It is a different engine. Some people say I am better, some people say he is. They are going to decide who is better in the moment and I think it's me."

What do the numbers tell us? If it's judged on goals or Champions League trophies, Ronaldo leads the way. If it's Ballons d'Or or total trophies, Messi shades it.

For some, Ronaldo nudged ahead when he helped Portugal lift the European Championship in 2016, but Messi has since guided Argentina to two Copa America titles and a World Cup.

"For me, Messi is the best player in history and Cristiano is the greatest goalscorer in history," Spanish football expert Guillem Balague says.

And Deco, who played with Ronaldo for Portugal and Messi for Barcelona, says: "They are special. They are totally different from the rest. It's not normal to be on this level all these years.

"Each year, there's a lot of players who do amazing things for a few years, but being almost 20 years is not normal."

Perhaps the beauty of the GOAT debate is that it may never be settled - and does not need to be.

"They have come to appreciate each as the co-stars of the football drama they have appeared in for the last 20 years," says Jonathan Clegg, co-author of a Ronaldo and Messi book.

The pair are seen by many as polar opposites.

The dribbler against the physical specimen. The shy genius against the arrogant ego. Pep Guardiola's Barcelona against Jose Mourinho's Real Madrid. Adidas against Nike.

But, while the narrative of their rivalry was undoubtedly built on their differences, the similarities between the two were striking - even from an early age.

Both players were raised in humble surroundings. Both left their homes at an early age to pursue their dreams - Messi to Barcelona from Argentina at 13 and Ronaldo to Lisbon from Madeira aged 12 - and both struggled with home sickness after leaving their family behind.

Wall Street Journal reporter Joshua Robinson, co-author of a Messi Ronaldo book, said: "Messi and Ronaldo are always portrayed as being so different but the things that forged them in their childhoods were incredibly similar.

"They both go to a place that says 'we will take your talent and make you even greater'. That promise feels appealing but is a huge gamble. That moment of total commitment is when they realise this isn't kid stuff any more. 'If I'm going to be the greatest of all time, this is where it begins.'"

But the biggest thing that brought the pair together was their unshakeable desire to succeed as they quickly built reputations as prodigious young talents.

"You could already see something different with Messi," said Xavi. "Not only the quality but the intensity with which he did things. There was an aggressiveness in attack that I had never seen before.

"Cristiano even gave him an extra push to be a better player."

Meanwhile in England, a teenage Ronaldo was signing for Manchester United from Sporting.

"I never met a young player so confident in knowing what he wanted," said Rene Meulensteen, who was part of the club's backroom team at the time. "The moment he came to Man United it was just a logical step of what he wanted to become - the best player in the world.

"Cristiano was a fast learner. I wanted to turn him from someone who could score a goal to a goalscorer. I made a three-minute video that just showed him scoring goals and also then the opportunities he didn't score. He embraced everything."

Fast forward to 2008 and the Champions League semi-final. Manchester United v Barcelona. Ronaldo v Messi for the first time.

"You could see already Messi was the best in La Liga and Cristiano was the best in the Premier League," said Spanish football journalist Guillem Balague. "To see those two kids on the pitch together was the sign of something to come."

Manchester United went on to win the trophy, Ronaldo lifted the Ballon d'Or and a rivalry for the ages had just begun.

"There is no doubt being in the same league changed everything," says Txiki Begiristain, director of football at Barcelona between 2003 and 2010.

Ronaldo's world record £80m move to Real Madrid in 2009 thrust the pair right into the centre of one of football's most intense club rivalries - Barcelona v Real Madrid.

And by the time Ronaldo left for Juventus in 2018, the pair had won five Ballons d'Or apiece. In the nine seasons the pair were together in Spain, Ronaldo had scored 450 goals in 438 games for Real. Messi, 471 in 476 games for Barca.

But it had become far more than just numbers. By now, it was personal - and the growth of social media meant the world was watching.

"For Cristiano it was Lionel Messi and for Lionel Messi it was Cristiano. 'I need to beat this guy'," said Begiristain.

"The Mourinho - Guardiola rivalry was like a mirror for the Ronaldo - Messi rivalry. And, as players, they knew that game-winning goals were their route to one-upmanship," added Spanish football writer Sid Lowe.

"We could watch it all on our phones. And in turn, the global exposure for the Messi - Ronaldo rivalry was now sky high, absolutely off the charts. Everything they did was must-see.

"It was on everyone's lips in the press box, newspapers and social media comments, Cristiano and Leo were hell bent on outdoing each other on the pitch. Their personal battle for supremacy was symbolised by the ongoing trophy battle between the clubs."

And what a battle it was. Messi and Barcelona undoubtedly won the La Liga honours, but the Champions League was dominated by Real Madrid and Ronaldo.

In 2012, Ronaldo inspired Real to their first La Liga title in four years but it was Messi who picked up his fourth straight Ballon d'Or award - much to his rival's disgust. He went to win four of the next five.

"There is a genuine animosity that begins to grow," says Robinson. "They didn't acknowledge each other that much, they hated comparisons.

"They could not tolerate if they were the greatest of all time, there could be another in their era, in their football league."

Deco adds: "I don't think there is something similar to what happened with Messi and Ronaldo at this moment because at the same time the two clubs, Barcelona and Madrid, were at the same level and fighting for the big trophies."

When Messi scored a 92nd-minute winner for Barcelona at Real Madrid in 2017, he removed his jersey and held it up the crowd.

"In the popular narrative, Cristiano had been the diva and Messi had been the humble servant of Barcelona, but this was the moment of Messi reasserting himself on the rivalry, saying maybe for the first time in his career 'look at me'," says Robinson.

Just a few months later, Ronaldo mimicked the celebration when he scored in the Spanish Super Cup at Barcelona.

Balague added: "If you needed proof of how much it meant to beat each other, those are the pictures."

The Messi v Ronaldo rivalry in Spain may have ended when the latter joined Juventus in 2018 - but the global, ongoing debate wasn't disappearing.

Ronaldo went on to return to Manchester United before moving to current club Al-Nassr in Saudi Arabia, while Messi moved to Paris St-Germain, before signing for Inter Miami in America's Major League Soccer - where he remains now.

Having seen their impact in Spain, for the first time the players' moves were as commercially motivated as they were for football reasons.

"Brand Beckham opened the door for what was possible and these two have kicked the door off its hinges," said Rob Pilgrim, head of sport at Meta for Europe, Middle East and Africa.

Italian football expert Mina Rzouki, added: "You are not buying sporting excellence, you are buying a whole one-man economy."

The numbers are astronomical and the rivalry continued.

In 2018, Ronaldo joins Juventus and sells 520,000 shirts in the first 24 hours. In 2021, Messi moves to Paris and shifts 150,000 shirts in just seven minutes.

Manchester United welcome Ronaldo back in 2021, and United's total sales of his shirt (£187m) are almost double those of Messi's at PSG.

On Instagram, Ronaldo has close to 700m followers, Messi has 500m. The most-liked picture in the history of that platform is the Argentine lifting the World Cup trophy, amassing more than 75m likes.

Financially, Ronaldo comes out on top - according to official channels anyway. The Portuguese has topped the Forbes Money List as the world's highest paid athlete for a fourth consecutive time, with $300m (£223m) in total earnings. Messi is third on the list with $140m (£104m).

The rivalry between the pair is only accentuated by the fact they are on either side of the sportswear rivalry. Messi with Adidas and Ronaldo with Nike.

"They have become enormous global brands and what is amazing about them is they have to say almost nothing to become the world's biggest pitch man," said Robinson.

Brands the world over threw unparalleled amounts of money to get the superstars on board.

And who could forget the iconic image before the 2022 World Cup in Qatar, with both footballers playing chess against each other on a Louis Vuitton suitcase, to launch the designer's latest campaign.

"It is known as the picture that broke the internet. It was a commercial masterstroke," said Melanie Ropp, from social media marketing company the Goat Agency. "The timing of it before the World Cup made it globally iconic."

So, back to where we started.

Robinson said: "Messi has nothing left to conquer. He has that one thing Ronaldo does not, a World Cup. Now the question is did Messi win this entire era of football?"

It has to be remembered this story is not over and, with Argentina and Portugal potentially meeting in the latter stages of this summer's World Cup, the final act could be about to play out. (BBC Sports)

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