A 1-0 win at home to Colo Colo on Sunday helped Catolica stay top on goal difference from O'Higgins with one Apertura match remaining.
O'Higgins stayed in touch with a 1-0 victory of their own, edging Union Espanola at home on Saturday.
At Catolica's Estadio San Carlos de Apoquindo, Ismael Sosa got on the end of a cutback on 50 minutes to score the only goal of the game.
For 12th-placed Colo Colo, Christian Vilches was sent off on 72 minutes as they failed to find an equaliser.
O'Higgins needed a 54th-minute goal from Pablo Calandria to overcome fifth-placed Union Espanola.
Matias Ramirez's winning goal four minutes before full-time saw Palestino to a 1-0 win at Deportes Iquique.
There were three red cards during Universidad Chile's 1-1 draw at home to Universidad Concepcion and Nublense were too good for Everton in a 1-0 win.
Union La Calera clinched a 1-0 victory at Huachipato and Audax Italiano struck twice in the second half to overcome Rangers 2-0.
Antofagasta and Cobreloa played out a 0-0 draw and Cobresal were held to a 1-1 draw by Santiago Wanderers.
