Monterrey's Rodolfo Pizarro accounted for the lone score of Wednesday night's contest when he headed home a pass by Dorlan Pabon in the fourth minute.
Stefan Medina then created danger with a pass into Cruz Azul's area in the ninth minute before Pabon sent a free kick over the crossbar in the 13th minute.
Goalkeeper Marcelo Barovero then was a key figure for fifth-seeded Monterrey in the second half, stopping one shot by Martin Cauteruccio and later thwarting Roberto Alvarado's run into the area in the 67th minute.
Late in the game, the Rayados missed two chances to put Cruz Azul in a deeper hole when a shot by Rogelio Funes Mori clanked off the woodwork in the 83rd minute and Pabon fired wide during second-half stoppage time.
Those Mexico City teams will play their first leg on Thursday night at Pumas' Estadio Olímpico Universitario and their second leg on Sunday evening at Estadio Azteca, America's home ground.
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