Gabriel 'Gabigol' Barbosa scored to earn Flamengo a 1-1 draw away to Brazilian rivals Internacional and a 3-1 victory on aggregate.
Flamengo – led by Jorge Jesus and boasting former Atletico Madrid star Filipe Luis – carried a 2-0 lead into the return leg in Porto Alegre.
Internacional gave itself hope just past the hour-mark when Rodrigo Lindoso glanced a header beyond Flamengo goalkeeper Diego Alves.
Gabigol – tipped to make his loan deal from Serie A giants Inter permanent – extinguished Internacional's chances with five minutes remaining after side-footing into an empty net, having received a simple pass from first-leg hero Bruno Henrique.
Flamengo, who claimed its one and only Libertadores title in 1981, will meet Gremio in the final four as they return to the semis for the first time in 35 years.
Meanwhile, Boca reached the semi-finals for the second successive year after its 0-0 draw at home to Antonio Valencia's LDU Quito midweek.
There were no goals scored in Buenos Aires but Boca still progressed 3-0 on aggregate after their comprehensive victory last week.
Daniele De Rossi came off the bench with 10 minutes remaining as six-time champions and last season's runners-up Boca await holders River Plate or Cerro Porteno in the semis.