Teofilo Gutierrez's late red card leaves the Copa Libertadores Superclasico evenly poised after River Plate won 1-0 at home.
A late penalty from Carlos Sanchez saw River Plate edge out arch-rivals Boca Juniors 1-0 in their Copa Libertadores last-16 first leg on Thursday but a red card to Teofilo Gutierrez will hurt next week.
Gutierrez was sent off with two minutes remaining at River's Estadio Monumental, marring the home side's victory, which came just four days after Boca won another Superclasico in the Argentine Primera Division.
River had the better of the scoring chances in their home leg but despite failing to gain a greater advantage ahead of next Thursday's trip to La Bombonera, coach Marcelo Gallardo was happy with his team's performance.
"We had to be decisive, aggressive and intense and tonight was all that. The victory was fair," Gallardo told River's website.
"Maybe we could have scored another goal. I'm satisfied … we had to win and not to concede goals."
In a tense match, Sanchez converted from the spot in the 82nd minute after Boca full-back Leandro Marin tripped Gonzalo Martinez in the penalty area.
But six minutes later River were reduced to 10 players when Gutierrez made late high contact with Guillermo Burdisso after the Boca defender had cleared the ball.
Gutierrez appeared to put his studs into Burdisso's thigh and the Colombian striker will miss the second leg.
In Uruguay, Racing Club stunned hosts Wanderers late in their first leg with Brian Fernandez scoring in the 87th minute to secure a 1-1 draw.
Wanderers had led from the 55th minute after Matias Santos scored.
In the last match on Thursday, Ecuador's Emelec won 2-0 at home over Atletico Nacional of Colombia thanks to goals either side of half-time from Emanuel Herrera and Miller Bolanos.