Both sides ended the game with 10 men with the Argentine striker at his scintillating best as the Ecuadorian side sunk an uncharacteristically disorganised La U
Los Azulgrana went ahead through Matias Alustiza but the game parity was restored by Matias Rodriguez after the hosts were reduced to 10 men. Carlos Ischia's side showed great character to regain and enhance their lead through Luis Checa and Alustiza before the visitors also had man shown red. Fidel Martinez rounded off a fantastic result for the Ecuadorian side with a powerful strike in the closing minutes.
The hosts were on top in the early stages as they enjoyed the lion's share of possession in the opening stages and shortly after Jorge Sampaoli was sent from the dugout the visitors lost their shape.
Quito recognised this and went ahead in the 30th minute as Alustiza latched onto Fredy Olivo's pass and smashed the ball past Jhonny Herrera.
The game was very bitty and no less than a minute later Olivo had handed the initiative back to the Chileans. A needless challenge in the middle of the park saw him dismissed for a second yellow card.
It caused Los Azulgrana to reshuffle tactically and with La U in the ascendancy, they could only hold them out until the 37th minute.
A sweeping move ended with Diaz clipping a beautiful pass to Rodriguez as he ghosted in behind Isaac Mina on the right of the box. He cut inside of the incoming challenges before picking his spot and bending a left-footed effort in at the far post.
Los Azulgrana were unperturbed and they continued to attack despite their man disadvantage, regaining the lead on the stroke of half time. A short corner was whipped in to the back post by Juan Paredes and Checa rose highest, heading the ball back across goal leaving Herrera no chance.
The story was no different as after the break and moments after volleying the ball wide, Alustiza made sure of finding the net in the 54th minute.
Alex Bolanos sprayed a superb diagonal pass that found the striker in acres of space on the right. He carried the ball into the area, ducked inside of Igor Lichnovsky all too easily, and fired his eighth goal of the tournament into the top corner.
It was an explosive game with an atmosphere to match and as tempers frayed, the sides were levelled up minutes after the goal. Osvaldo Gonzalez had had a torrid night and he was shown a red card for leaving a foot in on Martinez on the hour.
Junior Fernandes had been quiet for much of the game and he had two good opportunities to grab another vital away goal for his side but he could not beat Marcelo Elizaga.
It was a problem that Los Azulgrana did not face. In the 83rd minute Martinez burst forward from midfield, exposing an over-committed La U as he hammered an emphatic finish into the top corner.
The pair will meet in Santiago next week for the return leg and the reigning Copa Sudamericana champions will need to win by at least three clear goals in order to progress to the quarter-final.