Defending 170/6 against a confident Knights line-up, pace star Adam Milne took out two key wickets cheaply in Daniel Flynn and Corey Anderson - who skidded a catch to Will Young, who then did well to snaffle it from an awkward position in the deep.
But Devcich proved stubborn, and by the time the Stags had brought themselves back into contention with late wickets, the equation could have gone either way. With Kelly seeing it like a melon, it went the way of the Knights, who slashed the Otago Volts' Georgie Pie Super Smash lead in the process to just two points.
Earlier, at a warm Hagley Oval, the Canterbury Kings powered to a six-wicket win over the Auckland Aces, veteran right-hander Peter Fulton and young Cameron Fletcher piling on a thunderous 94 for the third wicket to all but seal the impressive win.
Fletcher (38 off 45) holed out with victory in sight, having played a support role for Fulton who whacked seven sixes in an undefeated innings of 76 from just 43 deliveries.
The win means the Kings leapfrog the Central Stags and the Wellington Firebirds into fourth place.
After having won the toss, the Aces were left ruing lost opportunities to close out their innings with more force, having melted from 121/3 to 132/9 after losing a calamitous six wickets in 17 balls to collapse. The turning point came when the two Colins - Colin de Grandhomme and Colin Munro - were dismissed in consecutive deliveries.
Kings veteran Andy Ellis did the late damage, finishing with four for 17 in a stunning display of death bowling; while for the Aces, Mitchell McClenaghan (three for 23) had been the key strike weapon, removing Ronnie Hira in the first over after having been hit for six first ball of the match. However, only Michael Bates would be permitted to join him in the wicket column as the Kings batsmen took control.
Teams now catch their breath for "Super Smash Sunday" on the weekend when all six sides will be in action at the same time as the race for the Finals intensifies.