Carlos Gonzalez's second-half goal was all that Pumas UNAM needed as the club beat Mexico City rivals Club America 1-0 on Sunday in Liga MX action.
The result at Estadio Universitario was Aguilas boss Miguel Herrera's first loss in 13 games against Pumas. The loss leaves the defending champions out of one of the eight playoff spots, but with a game in hand.
For Los Universitarios, the victory provided some salve after America had humiliated them 7-2 on aggregate in last season's Liguilla semifinals.
America had the best scoring opportunity of the first half, but Edson Alvarez's header off Renato Ibarra's attempt was handled by Alfredo Saldivar.
Less than three minutes after the break, Gonzalez buried a cross off a corner-kick past goalkeeper Agustin Marchesin for the only score of the day.
Nicolas Benedetti, coming on as a substitute for Herrera's side, missed a potential equalizer in the 60th minute after finding space and seeing his shot bounce off Saldivar's right leg.
Former Pumas star Nicolas Castillo, who signed with America in January following a stint at Benfica, had a relatively quiet day amid the jeers from the home side supporters.