River Plate at Internacional headlines three Copa Libertadores bouts between Argentina and Brazil

Lionel Messi and Cristiano Ronaldo have spoiled the world of football. It is not normal for anyone to stay at the top, year after year, for this length of time. In the normal run of things, the old adage applies -- it might be hard to reach the top, but staying there is even harder.

In few places does this apply better than in the Copa Libertadores, South America's version of the Champions League. Real Madrid are not going to win this season's European crown -- but they managed it in each of the last three years, and in four of the last five.

In South America such a feat is much harder to perform. Success puts players in the shop window and teams, especially winning ones, are often broken up quickly. Cycles are shorter and no one has retained the Libertadores title since Boca Juniors won it in both 2000 and 2001. No one has even reached two consecutive finals since Sao Paulo won in 2005 and were runners-up the following year.

Recent changes should, theoretically, have made it easier to retain the crown. The Libertadores now runs throughout the year, rather than being squeezed into the first semester. It means that the gap between the end of one version and the start of another is considerably shorter, which should make it easier to maintain continuity. The first winners of this long format were Gremio of Brazil in 2017 -- who last year were just a few seconds away from making it through to another final.

- Wright Thompson: Boca vs. River: The Greatest Game Never Played

They fell to River Plate, who then beat Boca Juniors in a controversial final to lift the title for the fourth time. Will there be a fifth this year? Their chances, at this stage, do not look especially good. In their first two games, River have been held to draws by, on paper at least, the weaker teams in the group, Alianza Lima of Peru and Palestino of Chile.

This Wednesday they travel to face heavyweight opposition, Internacional of Brazil. With two wins in the bag, Inter have six points while River have only two. A Brazilian victory would open up a seven-point gap with just three games to go, and if one side wins both matches between Alianza and Palestino, it would leave River struggling to finish second in the group. Defeat in Porto Alegre on Wednesday, then, would open up a genuine possibility of a first-round elimination -- a humiliation River have not experienced for a decade.

Coach Marcelo Gallardo is without the services of last year's playmaking heroes. Gonzalo "Pity" Martinez has moved north to join Atlanta United while Colombian Juan Fernando Quintero has suffered a serious knee injury. Someone will have to step up if River want to survive the group, never mind hold on to their crown. In the opening game away to Alianza, it took a superb last-minute free kick from 19-year-old Cristian Ferreira to rescue a point. At the weekend in a league game away to Talleres, it was another attacking midfielder, Nacho Fernandez, who came up big, tipping the balance in a 2-0 win that almost guarantees River's place in next year's edition of the tournament.

But, for Wednesday's crunch game in this year's competition, it seems that Argentina international goalkeeper Franco Armani has been ruled out. He picked up an injury against Talleres and it will be fascinating to see whether qualifying for the 2020 Libertadores comes at a price of jeopardising this year's campaign.

For Internacional, too, this is a big game. They have two Libertadores titles to their credit, but recently went through hard times, and were relegated to Brazil's second division -- a massive blow to the pride of a big club. They bounced straight back and last year, in their first season back, they exceeded all expectations by finishing third in the table. They were not reckoned to be Brazil's strongest candidate for this year's title, but they have started off with two wins. How good are they? Some still doubt them, but Wednesday's game will be a major test of their quality. They also hope to have their Argentine playmaker Andres D'Alessandro back, a former River Plate idol who is racing to be fit in time.

The fact that D'Alessandro has spent so long in Brazil is significant. The superior financial power of the Brazilian clubs is easy to identify; D'Alessandro is just one of a number of high-profile Argentines who have made the move from Argentina to Brazil. Conversely, there are no big-name Brazilians making a living in Argentine football.

And yet Brazil has supplied just one finalist in the last five years. The Libertadores has turned into a battle between Brazil and Argentina -- the two countries have supplied 13 of the last 16 quarterfinalists -- and it is a fight where Argentina have enjoyed the upper hand. Will that change this year? This week will give up some evidence, as in addition to the Inter vs. River clash, there are two other intriguing meetings of the South American superpowers.

First, Brazilian champions Palmeiras travel to face 2014 champions San Lorenzo. Palmeiras have won both games so far, but an ultra-cautious approach on Argentine soil proved their undoing in last year's semifinal against Boca Juniors. Will they be bolder against opponents in poor domestic form?

In the other, Boca Juniors travel to face Athletico Paranaense -- a match taking place in an especially interesting group. With Tolima of Colombia also looking dangerous, neither side can afford to slip up. At least, though, both teams have already chalked up one win this year -- which is more than can be said for the reigning champions. No doubt about it, Internacional against River Plate is the game of the week in the Libertadores.