Rookie Treena Limgengco fired 20 points, including five in the fourth quarter, as the Lady Eagles survived outright elimination in the Final Four and kept their title-retention bid alive.
University of the Philippines earlier stunned elims topnotcher University of Santo Tomas, 72-64, in the other semis encounter and likewise force a knockout duel for the other finals berth.
"I have to give it to the players, they want to extend the series," said Ateneo coach John Flores, who steered the school to its first womens crown last season.
The two knockout matches will be played Sunday.
Cassy Tioseco chipped in 16 points and plucked 19 rebounds for Ateneo as she boosted her bid in the MVP race, paced by last years MVP Merenciana Arayi of Adamson.
Diana Jose led the Lady Tams with 30 points, including a season best nine triples but failed to get enough support from her teammates. Joey Villar