America beat Toluca 4-3 in a thriller at the Azteca to join Atlas, who beat Apertura champions Monterrey 1-0 on Saturday, and San Luis on 13 points in a three-way lead of Group Two.
Pumas, the only unbeaten team after eight matches whose old-fashioned 4-2-4 system is paying rich dividends, scored through five different players.
They were two up through Paraguay defender Dario Veron and midfielder Israel Castro when UAG had defender Gustavo Cabral sent off in the 66th minute for a second booking.
Pumas conceded a goal three minutes later to UAG's Israel Lopez before scoring three more in a late flurry through forwards Javier Granados and Dante Lopez and substitute Carlos Trevino.
With 18 points, Pumas are three ahead of Morelia, who beat Atlante 2-1, and four in front of Cruz Azul, held 1-1 at Queretaro on Saturday, in the group standings.
America, one of Mexico's most popular teams, recovered from three defeats in a row, two in the league and one in the South American Libertadores Cup.
Midfielder Angel Reyna gave America a 1-0 half-time lead before an exhilarating start to the second half produced four goals in six minutes with the pick being Argentine playmaker Daniel Montenegro's shot from outside the box.
Defender Diego Novaretti scored twice for Toluca, who had substitute Luis Arias sent off for kicking Brazilian Rosinei with 14 minutes remaining.
Santos Laguna, runners-up in the Apertura in the first half of the season, lost 1-0 on Friday away to promoted Necaxa under new Argentine coach Diego Cocca, who has taken over from sacked compatriot Ruben Romano.
It was Cocca's second defeat in charge of Santos after a midweek loss to Cruz Azul in the Concacaf Champions League.
Santos, in Group One, are equal on 10 points with Guadalajara, Monterrey and Necaxa. They are all seven points behind leaders Tigres UANL, who beat Puebla 3-0 on Saturday.
The top two in each of the three groups and the two next best-placed teams in the overall standings after the 17-match round robin phase qualify for the quarter-finals.
