America and Monterrey upset in openers
Colombia midfielder Yulian Anchico, who joined from Independiente Santa Fe in his homeland after New Year, put Pachuca ahead in the 24th minute of Sunday's match and Paraguay striker Edgar "Bird" Benitez secured the win in the 66th.
It was an encouraging result for Argentine coach Pablo Marini's side after their disappointing Club World Cup last month in Abu Dhabi where they finished fifth.
Cruz Azul, favourites to win the Apertura championship in December before their shock quarter-final elimination by Pumas UNAM, crushed Estudiantes UAG 4-1 on Saturday with a fine performance from new Chilean signing Hugo Droguett.
Midfielder Droguett, who joined Cruz Azul from Mexico's Morelia, laid on goals for Argentines Emanuel Villa and Cristian Gimenez in the opening quarter of an hour with crosses from the left wing.
Estudiantes pulled one back with a penalty in the 18th minute converted by Argentine Mauro Cejas but Uruguayan Marcelo Palau, another new signing from fellow first division Puebla, restored Cruz Azul's lead before half-time. Substitute Gonzalo Pineda completed the rout with a late penalty.
"What we did was pathetic, we were very tame, lacking in conviction and character," Estudiantes coach Jose Luis Sanchez told reporters.
San Luis beat Monterrey, winners of the Apertura in the first half of the season, 2-0 with goals from defender Osmar Mares in the 73rd minute and forward Juan Carlos Medina, who chipped goalkeeper Jonathan Orozco in added time from Peruvian substitute striker Wilmer Aguirre's cross.
The champions had defender William Paredes sent off three minutes from time and coach Victor Manuel Vucetich dismissed to the stands a minute later.
"We weren't at our best, I think we didn't carry through moves we usually do. We've started with a defeat we don't like but we also know we have a good enough team to do well," said Vucetich, who complained of a very short break before the Clausura.
In another high scoring match on Saturday Atlas, who finished bottom of the standings in the Apertura, overran Morelia 5-0 with midfielder Gerardo Espinoza scoring twice.