After a 1-0 win at San Lorenzo a fortnight ago, River Plate started the second leg at their Estadio Antonio Vespucio Liberti in the box seat to win the two-legged tie but had to fight hard to secure a scoreless draw and an aggregate triumph.
San Lorenzo hit the woodwork twice and saw Martin Cauteruccio head the ball over the bar from point-blank range in the second half, although River also wasted opportunities to clinch victory late on.
The visitors' best chance to score came three minutes after half-time when Leandro Navarro's free-kick was parried onto the bar by River goalkeeper Marcelo Barovero, with the ball bouncing down and out without crossing the line.
With six minutes remaining, San Lorenzo hit the post again with a header before River conceded a corner kick.
From the resulting corner, Cauteruccio got free while standing at the back post but headed the ball over the bar when it would have been easier to score.
Just three minutes later and River's central defender Eder Alvarez shot over the bar after an overlapping run down the left, while in second-half stoppage time, Manuel Lanzini burst forward on the counter-attack for the home team, threaded the ball to Osmar Ferreyra but the latter's strike was saved by San Lorenzo's Sebastian Torrico.
Despite those missed chances, River held on for the aggregate win to set up a last-16 clash with Ecuador's LDU Loja.
San Lorenzo finished the game with 10 players on the field after Pablo Alvarado was sent off in the 90th minute.
