Gustavo Morinigo's Nacional Asuncion and San Lorenzo put themselves on track for the semi-finals of South America's premier club competition with 1-0 victories against Arsenal and Cruzeiro respectively in their opening legs.
Nacional Asuncion had edged Velez Sarsfield en route to the last eight and the Estadio Defensores del Chaco played host to Argentinean opposition yet again on Wednesday.
It proved to be a successful night for the Paraguayan giants, who will take a slender lead into the return leg thanks to Derlis Orue's 36th-minute effort, which ended the team's run of back-to-back draws.
The 25-year-old midfielder slid the ball through the legs of Arsenal goalkeeper Cristian Campestrini from a tight angle nine minutes before the break.
Arsenal, who are unbeaten in six games at home in the Copa Libertadores, will be without Ivan Marcone for the second leg after the midfielder was sent off deep into injury time.
Meanwhile, San Lorenzo got the better of Brazilian champions Cruzeiro at the Estadio Pedro Bidegain in Argentina.
San Lorenzo extended their unbeaten run to seven games in all competitions following Santiago Gentiletti's second-half header.
The opening 45 minutes failed to produce a goal but the home side's towering defender was on hand to guide his header past Fabio just after the hour-mark as Cruzeiro slumped to their first defeat in 13 games.
The return legs will be played on May 14.
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