Ronaldo arrives as the talismanic leader of the newly-crowned European champions, Real Madrid, off the back of a year's football in which he has comprehensively established himself as the world's best player.
Messi, as the superstar whose crown Ronaldo pinched, remains consistently the finest player of the last four years, however, having scored eight goals more than Ronaldo since the curtain came down on the last World Cup.
Not that Ronaldo will be too upset: he has 219 goals to Messi's 227 in that period.
Such staggering records bring huge expectations in themselves (it says everything that Messi's 2013-14 season is considered 'bad', despite him scoring 41 times in 46 matches for Barcelona).
But as if that sheer weight of expectation isn't enough, both men have further pressures on them.
Ronaldo, for example, has to deal with the fact that there are no obvious world class players in the squad aside from him. He will be expected to drag the team through the tournament almost single-handed - a feat which he managed in the play-off against Sweden, but which will surely prove too much for the duration of an entire tournament, against stronger and more confident teams.
Messi's battle, by contrast, comes from the fact that he has so many excellent team-mates. The Argentina side has never been built around him in the same way as the Barcelona team has - and nor should it, considering that the likes of Sergio Aguero, Gonzalo Higuain and Angel di Maria are in the squad. But the different set-up has meant Messi has consistently failed to produce the goods for Argentina in the way that he does for Barcelona. This World Cup, his first on the soil of his native South America, is probably his best, and his last chance to show what he is capable of on the biggest stage of all.
For all those psychological pressures on both men, however, there is an even bigger problem for them to deal with: complete and utter exhaustion.
As this graphic from our friends at WhoScored.com shows, no outfield players at the World Cup will have been on the pitch for longer than Ronaldo and Messi over the past four years:
Ronaldo has made more appearances (181) in the top 5 European leagues, Champions League and Europa League since the South Africa World Cup than any other player bar goalkeepers Petr Cech (189) and Steve Mandanda (184).
On top of those appearances, Ronaldo has also contested 45 Copa del Rey games and another four Supercopa matches as well as international appearances, including games in November in which he almost single-handedly dragged Portugal through the play-offs to qualify for Brazil 2014.
Messi started the season poorly as he was held back by injuries, but was actually reinvigorated after time on the treatment table. Indeed, he actually tops the scoring charts in Europe's top five leagues so far in 2014, with 20 goals.
Perhaps his winter lay-off provided the rest he needed to avoid burnout at the World Cup - though he'll still have all those other issues to deal with.
As for Ronaldo? Playing the Champions League final meant that he had almost no time off between the end of his season and the start of the World Cup build-up, so perhaps it's not surprising to see that his collection of injuries and niggles seems to be getting worse all the time.
Minutes played statistics courtesy of WhoScored.com
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- Cristiano Ronaldo
- Lionel Messi