A Carlos Sanchez penalty with 10 minutes to go gave River Plate a 1-0 win over their traditional Argentinian rivals Boca Juniors in the first leg of their Copa Libertadores last-16 tie.
Played in front of a crowd of more than 60,000 River fans at the Monumental stadium, with no visiting fans allowed due to violence in stadiums in Argentina, the derby was very closely fought but the hosts were able to secure a deserved victory.
In the first half River's midfield put pressure on opponents Fernando Gago and Nicolas Lodeiro, preventing them from creating any clear chances, while the hosts dominated.
Three good chances went begging for River as Teo Gutierrez, twice, and defender Ramiro Funes Mori failed to find the net.
Boca had their best chance only 20 seconds after half-time, but goalkeeper Marcelo Barovero denied Jonathan Calleri in a one-on-one situation.
The visitors then came back into the game but in the 81st minute, Gonzalo Martinez was fouled in the box by Leandro Marin and Sanchez netted the penalty.
The one sour note for River came when Gutierrez was sent off with two minutes remaining, but they will take a vital lead into next week's second leg at La Bombonera.
Emelec, from Ecuador, took a 2-0 first-leg lead over Colombia's Atletico Nacional with goals either side of half-time from Emanuel Herrera and Miler Bolanos.
In Montevideo, Argentina's Racing Club salvaged a 1-1 draw away to Uruguayan side Wanderers thanks to a goal in the 87th minute.
Matias Santos had opened the scoring in the 54th minute for Wanderers, but Brian Fernandez levelled late on.