Jockey Darragh O'Keeffe produced a never-say-die ride on A Plus Tard (15-2) in a dramatic renewal of the Savills Chase at Leopardstown.
Henry de Bromhead's runner had plenty of ground to make up on Melon and Kemboy, but started to close at the final fence.
The latter had mastered his stablemate and looked set to land a one-two for trainer Willie Mullins.
However O'Keeffe, riding in his first Grade 1 race, had other ideas and his mount stormed home to take the prize by half-a-length.
"It's unbelievable," he told ITV Racing.
"Saturday declarations when I was looking down to see how many rides I had and then I came onto the Savills Chase and I saw my name down for A Plus Tard, it was crazy.
"He wasn't a horse just making up the numbers in a great race, he had a live chance. People were doubting whether he would stay or not but he's an older horse now and he races behind the bridle.
"He jumped great and I'm very grateful to Henry de Bromhead, it's my first winner for him and Cheveley Park as well for giving me the opportunity.
"It's bizarre but I'm over the moon."
The favourite Minella Indo, a stablemate of the winner, fell at the first fence past the grandstand on the final circuit.
Last year's hero Delta Work, with Sean Flanagan replacing the sidelined Jack Kennedy, departed at the next obstacle.
O’Keeffe was recording his second notable win of December, as he also struck in the Caspian Caviar Gold Cup at Cheltenham on Chatham Street Lad.
A Plus Tard is 10-1 for the racecourse's showpiece event, the Magners Cheltenham Gold Cup in March, with bookmakers Paddy Power.
