Defending champions River and Boca are going head-to-head in a rematch of last year's controversial final, which the former claimed 5-3 on aggregate.
The second leg of the all-Argentinian 2018 decider had to be rearranged and moved to Real Madrid's Santiago Bernabeu in December after the match was twice suspended in November due to an attack on Boca's team bus by supporters.
Pitted against each other in the semis, hosts River drew first blood thanks to Rafael Santos Borre's seventh-minute penalty and an Ignacio Fernandez goal in the 70th minute on Tuesday.
Boca's woes were compounded by a stoppage-time red card to Nicolas Capaldo, who was initially shown a yellow before the referee consulted VAR in the 97th minute.
River made a bright start at El Monumental in Buenos Aires and they were rewarded when the referee pointed to the penalty spot after consulting VAR following a goal-mouth scramble in the third minute.
Boca – playing without Daniele De Rossi – had taken the ball up to the other end as play continued when the referee eventually viewed the replay for the incident involving River, with Borre converting the penalty four minutes after the initial scramble.
A stunning long-range effort from Alexis Mac Allister in the 17th minute almost restored parity for Boca, but River goalkeeper Franco Armani produced a great save at full stretch to tip the ball over the crossbar.
River were in control of the first half and Borre came close to doubling the lead, however, his volley was saved by Esteban Andrada approaching half-time.
For all of River's dominance, they nearly conceded three minutes later but Capaldo wasted a gilt-edged opportunity – the midfielder somehow blazing over after being played in by Ramon Abila following a great counter-attack.
River almost made it 2-0 out of nothing, with Gonzalo Montiel's mis-hit cross leaving Andrada scrambling as the 55th-minute delivery came back off the bar.
Some desperate defending prevented River from scoring again following a low cross into the six-yard box – the ball deflecting agonisingly wide of the post on the hour.
But River got in behind Boca's defence again and they finally managed to double their lead with 20 minutes remaining after Fernandez and Matias Suarez combined superbly as the former tapped home.