Cruz Azul held in home leg

No quarter given ... Cruz Azul's Israel Castro is brought down by Libertad's Pablo Velazquez (AAP)

Mexico's Cruz Azul drew 1-1 with Libertad of Paraguay in the first leg of its Copa Libertadores Round of 16 tie.

The match started badly for Cruz Azul, as Libertad's Pablo Velazquez unintentionally knocked out Jose de Jesus Corona with his left knee as the goalkeeper raced off his line to deny the attacker an opening.

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 to give his side the edge ahead of the second leg.

Velez Sarsfield took a step towards the quarter-finals with a 1-0 win over Atletico Nacional in Colombia.

Ricardo Gareca's side played solid football to protect the lead it claimed in the ninth minute at the Estadio Atanasio Girardot, when Ivan Bella scored a left-foot strike from outside the box.

Atletico, which 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.

It was the first victory on Colombian soil for Velez in continental championships.

"The worst thing we can do is to think the tie is finished," coach Gareca told reporters.

"In Buenos Aires, we'll have to make a big effort. We have claimed a very important win but it is not decisive."