Monterrey, who appeared to have stopped the rot last weekend with a win and their first three points, are separated from the foot of the table only by promoted Necaxa.
Necaxa's fourth successive defeat, 1-0 to Queretaro, led to the sacking of Argentine coach Daniel Brailovsky, announced in a brief statement by the club later on Sunday.
"We're aware that wins are always urgent, especially with a team normally used to winning and putting on good matches. We need to reach the level of previous tournaments," Monterrey assistant coach Carlos Barra told reporters.
Pumas, who fielded a four-man front line, scored through strikers Juan Carlos Cacho with his fifth goal in four matches, Argentine Martin Bravo and a superb volley by Javier Cortes while their goalkeeper Alejandro Palacios had a fine game.
Chile's Humberto Suazo scored a first-half equaliser for Monterrey with a penalty and Paraguayan midfielder Osvaldo Martinez a late consolation after Pumas had dominated the second half at the Olimpico.
AMERICA WIN
America's decision to sack coach Manuel Lapuente and bring back Chilean Carlos Reinoso for a third spell at the club he also played for in the 1970s paid immediate dividends.
The Mexican giants ended Atlas's fine start to the season with a 2-0 away victory on Saturday with goals in the final 19 minutes from Argentine-born former Mexico forward Matias Vuoso and substitute Angel Reyna.
"I think we're on the right track and we deserved the win because we never gave up on it. I'm going to be very demanding and America must be a team that always goes out to dictate play on any pitch and against any rival," Reinoso told reporters.
Atlas, however, remain the team with the most points, nine after four matches. Pumas, San Luis and Toluca, now the only three unbeaten sides, are a point behind.
Guadalajara, who rival America as the country's biggest and most popular club, lost 1-0 to Estudiantes UAG in the western city's derby with an added-time goal by substitute Eduardo Lillingston on Friday night. They are equal with Monterrey on three points.
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