A dress-rehearsal for the Copa Libertadores semis?

On Sunday River Plate meet Boca Juniors in Argentina, while Flamengo face Palmeiras in Brazil.

These are huge league games involving giant clubs who are fighting for the domestic title. And by strange coincidence, they may also serve as rehearsals for the semifinals of the Copa Libertadores, South America's Champions League.

The last four of the competition will be defined over the next three days and, on the evidence of last week's quarterfinal first legs, the most likely scenario in the next round is that River Plate will meet Boca Juniors, while Flamengo will face Palmeiras.

The Argentine clash looks especially likely. After all the controversy over the meeting between the two historic rivals in last year's final, it would be the home and away tie that some are eagerly awaiting, and others are dreading.

- Boca vs. River: The greatest game never played
- Vickery: Boca, River must beat tough foes to set up Superclasico

Boca would seem to have at least a foot and a half in the semifinals. Their first leg result, a 3-0 win away to Liga of Quito, was a triumph for coach Gustavo Alfaro, who went into the match under pressure after shock elimination from the domestic cup. He took a bold decision -- to fly up to Quito two days before the game. Normally, teams visiting the Ecuadorian capital go first to Guayaquil, at sea level, and make the move to Quito as close to kick off time as possible. This is because of the altitude -- at 2,800 metres above sea level the unacclimated player suffers from the lack of oxygen. Leaving it to the last minute may lessen the effects. But Alfaro's point has merit. A training session in game conditions allows the players to judge the conditions and adjust their game accordingly. And Boca played a superb match. Barring a disaster in Buenos Aires, a semifinal place is theirs.

River Plate have a bit more work to do. Last week they were pleasingly fluid in attack at home to Cerro Porteno of Paraguay, but they needed VAR assisted penalties to score their two goals. The Paraguayans had limited attacking opportunities but showed themselves capable of causing problems in the air. On Thursday, they will have a packed stadium behind them: "La Olla" the pressure cooker, where they have a 100 percent record in this year's competition. River also have problems in defensive midfield. Enzo Perez is suspended and Leo Ponzio is injured. Cerro Porteno will surely come out with all guns blazing, but River's capacity to score an away goal makes them the clear favourites.

There is a similar scenario in Wednesday's all-Brazilian tie between Flamengo and Internacional. Last week, Flamengo struck late at home in Rio de Janeiro to open up a 2-0 lead. Inter were strangely cautious in an ill tempered affair. Now, back In Porto Alegre they are obliged to chase the game, and will surely employ more attacking pace in a bid to get behind Flamengo's high defensive line.

New league leaders, Flamengo bristle with attacking talent, and should be able to clinch matters with an away goal. But they, too, have a problem in central midfield. Willian Arao is suspended, and his usual partner, the Colombian Gustavo Cuellar, wants a move away and has fallen out with the club. Cuellar was separated from the squad at the end of last week after announcing that he did not want to play. But his services are now required, and he has quickly flown down to Porto Alegre to take part in the game.

And the round kicks off with the other all Brazilian clash. Palmeiras took a giant step into the last four when they came back from Porto Alegre with a 1-0 win against 2017 champions Gremio. They now defend their lead in front of their own fans in Sao Paulo -- but not in their own stadium. The hugely impressive Allianz Parque, by general consensus the best of Brazil's new grounds, is not available. A concert is taking place there, so they have to move to the municipal Pacaembu stadium, for so long the home of historic rivals Corinthians. Can Gremio take advantage?

If so, they may look to attacking midfielder Luan, chosen as the best player in the Americas in the team's triumphant 2017 year. Since then, though, his career has stalled -- disillusioned, perhaps, by the lack of interest from major European clubs. He has lost his place in the starting line-up, and did not even appear off the bench last week as the team failed to break down an organised Palmeiras defence. But he looked sharp at the weekend, when Gremio fielded a reserve side in a league game. Luan made his case for a recall. Now can he stop the weekend's big matches being rehearsals for the Libertadores semifinals?