When asked if a failure to win a World Cup would be a blot on his career, Ronaldo said: "Honestly, I don't think so. I achieved everything I dreamt of in football. If you ask me if I want to win the World Cup... of course. But if I had to end my career right now, I would be very proud. I never thought my career would be so beautiful."

Likewise, Messi believes his team should not be fancied to win and rates Brazil as the tournament favorites. But that hasn't stopped him from dreaming of being in the winner's circle in Moscow.

"It is a lifelong wish and every time that a World Cup approaches the dream becomes a little stronger," Messi said during an interview on Argentine television in April. "The 2014 final was painful because we came so close. Winning a World Cup is what we all want."

So can Ronaldo or Messi add an elusive World Cup medal to their already crammed trophy cabinets? Even with a motivated and in-form Ronaldo driving them on, the task looks beyond Portugal, given the team's advanced average age and lack of quality, especially in midfield. Realistically, the European outfit would do well just to reach the quarterfinals.

Argentina are a more formidable unit, especially with their vast attacking resources. However success depends largely on whether Sampaoli can find quick and effective solutions to several dilemmas like who will play in central defense and who will be the goalkeeper.

He will also have to consider if he'll play an extra striker to compensate for his team's attacking midfield deficiencies. Even if he finds solutions for all these problems, he'll still need Messi to produce yet another individually sublime performance over the course of the tournament. But that goes without saying.