Adrian Aldrete's long diagonal pass from his own half saw players from both teams compete to head the ball 40 yards from the Montreal goal.
The ball fell to Antuna wide open racing in from the right side. Antuna went in one-on-one with Montreal goalie Sebastian Breza and fired home a 12-yard shot.
The visitors never put a shot on target against Cruz Azul goalie Jose Corona, while Breza was credited with one save.
The series' second leg is scheduled for March 16 in Montreal.