Ten-man Velez Sarsfield clinched a 1-0 win at Argentinian rivals Newell's Old Boys in the first leg of their Copa Libertadores last-16 tie.
During the first half, Newell's enjoyed the best scoring chances and were unlucky not to break the deadlock.
Agustin Allione got the only goal of the game in the 62nd minute and Velez held on even after Fabian Cubero was sent off seven minutes before the end.
In the seventh minute, home midfielder Pablo Perez crashed an outstanding volley from outside the box into the crossbar, then Argentina international Maximiliano Rodriguez could not convert when through one on one with Uruguayan goalkeeper Carlos Sosa.
Gerardo Martino's squad maintained control of the ball and they hit the woodwork again when forward Ignacio Scocco's free-kick struck the bar.
At the beginning of the second half, Perez got his head to a corner but defender Emiliano Papa blocked the ball on the line.
However, near the hour mark, Velez scored on a counter-attack, when Emanuel Insua found Allione, who beat goalkeeper Sebastian Peratta to fire Velez ahead.
Sosa saved well from an excellent Santiago Vergini shot, then Cubero was given his marching orders after picking up a second yellow card.
"We deserved a better score," said Paulo Cruzado of Newell's. "We tried to look for the goal every time we could. We did everything possible, but it was not enough. It is sad, but now we have to think about the next clash."
Man-of-the-match goalkeeper Sosa said: "It is a very important triumph, but
nothing has been decided yet. They are a well-balanced squad, that play nice football. In our venue, it will not be easy."