With the fifth set tied at 8-8, the Junior Tiger Spikers scored five unanswered points, capped by a Paul Colinares block to snatch a 13-8 lead that put the fight away from the Bullpups, who lost seven players from last year's championship run.
Rey De Vega exploded for 29 points, including 22 attacks, three blocks and four service aces while rookie Jay Dela Noche chipped in 14 points for the Junior Tiger Spikers, who ruled the tournament two years ago.
In the other opening day matches, Far Eastern University-Diliman overcame a slow start to beat Adamson University, 23-25, 25-15, 25-22, 25-23, while University of the East scored a convincing 25-22, 25-18, 25-18 victory over Ateneo.
Jerold Talisayan and Rans Cajolo each tallied 15 points, while Angelo Lipata and France Racaza chipped in 12 and 10 markers, respectively, for the Baby Tamaraws, who had a huge 12-5 advantage in the service area.
JP Mangahis came through with 12 points while team captain Francis Babon chalked up nine markers for the Junior Warriors.
Alexis Mendoza led the Blue Eaglets with nine points built on seven attacks and two blocks.