Santiago Silva scrambled home a 90th minute goal to give six-times champions Boca a 1-1 draw at Fluminense which sent them through 2-1 on aggregate.
In the other match, Paulinho headed an 87th minute winner for Corinthians, the only goal of a dour, fear-ridden two-leg tie against their fellow Brazilians Vasco da Gama.
Fluminense, missing playmaker Deco and striker Fred through injury, still managed to take a 17th minute lead when defender Carleto scored with a free kick which took a deflection and left goalkeeper Agustin Orion stranded.
The Brazilians managed to nullify Boca playmaker Juan Roman Riquelme as the visitors were reduced to launching long balls forward for Silva to chase.
Beaten finalists four years ago when they overcame Boca in the semi-finals, Flu were close to settling the tie when Thiago Neves was denied by Orion and Rafael Moura shot wide from an excellent position in the next attack.
Instead, Boca went through with the last move of the game as Diego Rivero's shot hit the post, goalkeeper Diego Cavalieri clawed it away only for Uruguayan striker Silva to force the ball across the line.
The Corinthians-Vasco tie produced more crunching tackles than clearcut chances and the hosts had coach Tite sent off just after the hour for protesting the referee's decisions.
Vasco had two good chances in quick succession in the second half when Diego Souza galloped clear of the Corinthians defence but had his effort saved by Cassio, then Nilton headed against the post.
Instead, Corinthians, who have never won the competition, snatched victory when Paulinho headed in from a corner in the 87th minute.
