Leon have struggled to recapture the form that saw them claim the Apertura title but they were good enough to hold Cruz Azul to a 0-0 draw.
Visitors Cruz Azul had a chance cleared off the line in the first half and Mariano Pavone's powerful close-range effort was somehow kept out in the second.
Pavone had another opportunity scrambled off the line during a second half in which former Barcelona defender Rafael Marquez - now with Leon - was sent off.
While Cruz Azul edge further clear at the top, Leon sit 11th in the table after winning four of 12 games in the Clausura campaign.
Cruz Azul have Atlante to thank after the fifth-placed side upset Toluca 2-1.
Roberto Gutierrez turned nicely in the area to put Atlante ahead on 32 minutes and their lead was doubled early in the second half.
Michael Arroyo's 54th-minute penalty made it 2-0 and Toluca were unable to respond until the dying stages, when Juan Salgueiro struck with a 30-yard free-kick.
Goals from Walter Ayovi and Enner Valencia saw Pachuca return to winning ways with a 2-0 victory at home to Chiapas.
Pumas UNAM were beaten for the second time in three league games after Dario Veron went from hero to villain in a 2-1 loss at home to Monterrey.
Veron had levelled for Pumas on 49 minutes but his own goal with 17 minutes remaining gave Monterrey three points.
Juan Rodriguez's brace was not enough for Santos Laguna as they drew 2-2 at Morelia, while America sit seventh after a 0-0 draw against Veracruz.
Guadalajara played out a 1-1 draw at Atlas, while Queretaro struck twice in the first half to win 2-0 at Tigres UANL.
Tijuana claimed a 3-1 win at home to Puebla.
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