Cabatu proved steadier down the stretch than the prolific duo of Kelly Williams and Arwind Santos, scoring five of his 20 points in the extra period, including the game-winner.
Game 2 is on Sunday at the same venue.
"He was really our first option and I’m happy he responded," said Rain or Shine coach Caloy Garcia, referring to the 21-year-old La Salle student.
Blessed with height and heft, a soft touch from the outside and the dribbling skills of a point guard, Cabatu used up nine precious seconds then launched the shot over the outstretched arms of Mark Isip that swished the net.
The former Archer, who also drained a triple in regulation that forced the extra period, whopped it up at center court, body-slammed with his teammates and pointed at the gallery where his father, former PBA cager and two-time PBL MVP Sonny Cabatu, was seated.
"I didn’t feel anything, I just took the shot," said Cabatu, member of La Salle’s champion squad in the UAAP two years ago.
Williams and Santos provided almost everything for Magnolia by combining for 35 points, 33 rebounds, five assists, five shot blocks and three steals but failed to get the job done at endgame. The long-limbed Santos, a force in the UAAP, even fouled out after he hacked Cabatu while attempting a three-pointer with 1:14 to go in the contest.
The Elasto Painters threatened to pull away early as they erected nine-point leads twice, the last at 26-17, midway in the second quarter.
But Williams and Santos struck back, fuelling an 11-2 blitz that knotted the count at 28-all before the Wellbest-owned franchise regained a 31-28 advantage at the half, thanks to Marvin Ortiguerra’s triple atop the key.
Earlier, Cebu hotshot Jett Latonio exploded with 20 of his 24 points in the second half as Granny Goose trounced a Joseph Yeo-less Harbour Centre, 87-81, to clinch its second straight third place finish.