RIO DE JANEIRO, Oct. 21 -- Bitter football rivals Brazil and Argentina will this week face off in two tantalizing battles for a place in the final of South American football's top club competition.

On Tuesday, Buenos Aires giants River Plate will host defending champions Gremio of Porto Alegre and on Wednesday Luiz Felipe Scolari's Palmeiras will also visit Argentina's capital for a clash with Boca Juniors in their Copa Libertadores semifinal first legs.

Return matches in Porto Alegre and Sao Paulo will be played the following week, with the winners over both legs earning a berth in the final, also to be played on a home-and-away basis on November 11 and 28.

River Plate and Gremio both have injury worries as they prepare for Tuesday's clash at the Monumental in Buenos Aires' northern districts.

The Argentine outfit received good news on the weekend when medical staff declared that midfielder Gonzalo Martinez has fully recovered from a hamstring strain.

However doubts hang over the availability of Argentina international goalkeeper Franco Armani, who has not played since suffering a quadriceps injury in early October.

"He has been feeling better these last few days but needs more time to fully recover," River coach Marcelo Gallardo said on Sunday. "I think he will be okay but we'll see in the next few days."

Gremio's fitness concerns are more pressing. Forwards Luan and Everton, the team's leading scorers this year, have both been ruled out of Tuesday's match by coach Renato Gaucho despite local media reports suggesting they could play.

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