Monarcas saw their lead atop the Mexican Clausura cut to one point last night following a 2-1 defeat at home to Toluca.
Early goals from Antonio Rios and Aureliano Torres put the visitors in control and, although Jorge Kalu Gastelum halved the deficit ten minutes before half-time, there was no way back for Monarcas.
Second-placed CF America moved within one point of the pace-setters but will rue a missed opportunity to go top after being held to a 1-1 draw at ten-man Club Tijuana.
The home team had Jose Sand sent off after only six minutes and looked to be heading for defeat when substitute Raul Jimenez opened the scoring for America after 62 minutes, but Duvier Riascos equalised a minute later.
fifa.com