Velez Sarsfield took a step towards the Copa Libertadores quarter-finals as they claimed a 1-0 win over Atletico Nacional in their last-16 first leg in Colombia on Tuesday.
Ricardo Gareca's side played solid football to protect the lead they claimed in the ninth minute at the Estadio Atanasio Girardot, when Ivan Bella scored a left-foot strike from outside the box.
Atletico, who changed coach at the weekend, replacing Santiago Escobar with Norberto Peluffo, found Velez keeper Marcelo Barovero in superb form in Medellin.
Velez could have widened the gap in the 54th minute with a penalty, but Gaston Pezzutti saved Juan-Manuel Martinez's spot-kick.
The Argentinian side, who won the Copa Libertadores in 1994, will make it into the next round with a draw in Buenos Aires next Tuesday.
It was the first victory on Colombian soil for Velez in continental championships.
Coach Gareca told reporters: "The worst thing we can do is to think the tie is finished. In Buenos Aires, we'll have to make a big effort. We have claimed a very important win but it is not decisive."
The match started badly for Cruz Azul, as Libertad striker Pablo Velazquez unintentionally hit Jose de Jesus Corona's head with his left knee and the goalkeeper was forced to leave the field.
Yosgart Gutierrez came on to replace Corona in the 14th minute and the Mexican team went ahead through Javier Orozco's goal in the 27th minute.
However, Velazquez headed a free-kick into the back of the net to level in the 70th minute.