Paraguayan outfit Nacional Asuncion took a 1-0 lead to Argentina and that was enough as they drew 0-0 at Arsenal to advance.
San Lorenzo won their first leg against Cruzeiro 1-0 and a 1-1 draw away from home saw them progress to the last four.
At Arsenal's Estadio Julio Humberto Grondona, the first half was made up largely of half-chances with the hosts unable to capitalise on a couple of opportunities.
Arsenal looked the more likely to score early in the second half before the visitors had late opportunities as the game opened up.
But the 0-0 draw suited Nacional Asuncion and they held on.
In Brazil, Ignacio Piatti gave San Lorenzo the perfect start to the second leg against Cruzeiro by opening the scoring in the 10th minute.
The attacker beat his marker inside the area before firing a powerful shot past goalkeeper Fabio.
Bruno Rodrigo gave the hosts hope in the 71st minute, heading in a Dagoberto cross from the left to make it 1-1 in the second leg.
But that was to be the only joy they would have as San Lorenzo advanced.
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