The two teams figured in a neck-and-neck battle practically throughout but the Stags operated better at crunch time, pulling off the crucial win to move on the threshold of a first title in four years.
With the decisive psychological edge and the momentum in the short series now on their side, the Stags go for the title clincher in Game Two tomorrow.
"We had a hard time breaking their defense in the fourth quarter, so we tightened up our own defense and we beat them in a defensive battle at endgame," said San Sebastian coach Turo Valenzona.
Indeed, the Stags put up a resolute defense while going to Clark Moore, Nuiranjam Alfad, Mark Macapagal and Roy Falcasantos for the needed baskets in the homestretch.
Falcasantos proved to be SSCs biggest hero in the end as the shifty guard scored on a penetration to break a 74-all standoff under a minute left in the clock, then nailed a charity later for the final count.
The Heavy Bombers, the elimination-round topnotchers, carried a 59-53 lead into the final quarter but wilted under pressure, yielding the game on several missed shots in the closing seconds.
Joel Finuliar, whos usually deadly from the outside, muffed a potential game-tying three-pointer from the right quartercourt with time down to seven seconds.
Earlier, after SSC surged ahead at 76-74 on Falcasantos drive, Ariel Capus took an ill-advised jumper with no JRU rebounders under the basket. With the loss becoming inevitable with SSC getting ball possession, the JRU crowd, disappointed by Capus booboo, started pelting debris onto the court.