The breakthrough goal in the 1-0 win came after 50 minutes when Riascos sped in from the right and slalomed between a pair of challenges on the edge of the penalty area before drilling a low 15-yard shot across goal and inside the far corner.
Toluca's hopes of an equaliser were hit in the 73rd minute when Antonio Naelson was dismissed for a second bookable offence after an unnecessary late challenge.
Tijuana lead Toluca by one point after 12 rounds of games.
Mid-table Jaguares condemned second-bottom San Luis to a 4-0 defeat.
Hosts Jaguares went ahead when Luis Rey scrambled a 10th-minute strike, before the same player turned in the second goal from close range 16 minutes later.
Jhon Cordoba passed home the third beyond Cesar Lozano, and substitute Franco Arizala wrapped up the emphatic home win six minutes from time.
Unam Pumas held off Monterrey to post a 3-2 home win, with goals from Eduardo Herrera and Efrain Velarde and an own goal from Jose Maria Basanta. Cesar Delgado and Hector Morales replied for Monterrey.
Morelia came from behind to draw 2-2 at Atlante.
Luis Venegas scored in the third and 17th minutes to put Atlante apparently in charge, but on a muddy pitch Miguel Sabah tucked in two close-range shots in the 82nd and 85th minutes to retrieve a point for Morelia.