Un De Sceaux saw off stable companion Douvan to claim victory in the BoyleSports Champion Chase at Punchestown.
Trainer Willie Mullins saddled four of the nine runners in the two-mile Grade One, with Douvan the 4-5 favourite under Paul Townend.
However, the admirable Un De Sceaux, ridden by the trainer's son, Patrick, fenced fluently out in front and passed the post two and three-quarters of a length ahead of the odds-on market leader.
A Toi Phil finished third, but was another seven lengths adrift at the line.
Patrick Mullins said: "I've been dreaming about riding that horse. When I was told I was riding him I was like Charlie when he got the golden ticket off Willie Wonka!
"I schooled him yesterday and even schooling him was some buzz.
"It took him a while to warm up into the race. He didn't jump or travel like he can and I was a little worried, but I think that was just the good ground.
"Once he'd warmed up I just let him at it and he runs horses into the ground."
He added: "Realistically I didn't think he was going to beat Douvan or Min on this ground over this trip, but he's a horse that always runs well, so you can never count him out."
That result saw Willie Mullins close the trainers' championship deficit to €226,714, having been over €521,000 behind Gordon Elliott at the start of the day.
Champion NH Trainer Title Update After Race 4, Day 1 -— Horse Racing Ireland (@HRIRacing) April 24, 2018
Gordon Elliott leads Willie Mullins by €226,714. Gordon Elliott: €4,782,939 Willie Mullins: €4,556,225. pic.twitter.com/trV2RMUcYQ
Draconien got Willie Mullins off to a flying start after springing a surprise in the Herald Champion Novice Hurdle.
The reigning champion trainer has significant ground to make up on Elliott in this year's title race, but reduced the deficit in the first Grade One contest of the week.
Mullins fielded four of the 10 runners and it was his 11-10 favourite Getabird who cut out much of the running in the hands Paul Townend.
However, he was a spent force from the home turn and was first claimed by Colin Tizzard's British challenger Vision Des Flos, before 25-1 shot Draconien came through under a confident Noel Fehily to claim victory by two and a quarter lengths.
Vision Des Flos was a clear second, with Elliott's Mengli Khan best of the rest in third.
Earlier, Just Wait And See gamely held off Blue Templar to win the Kildare Hunt Club Fr Sean Breen Memorial Chase for the Ladies Perpetual Cup, the first race of the 2018 Punchestown Festival.
Ridden by Billy Gleeson and trained by his father, John, the victorious 16-1 chance hit the front inside the final half a mile.
But Derek O'Connor conjured up a late rally from Blue Templar, having been under pressure from some way out, and only went down by a head in a thrilling finish.
John Gleeson said: "I only have a handful of horses to keep an interest in the game. This has given me the biggest buzz I've ever had in my life."
Billy Gleeson said: "We've had the horse since he was four.
"He's a real fun horse - a very clever jumper - and God knows how many fences he's jumped in his life."
True Self (12-1) showed a smart turn of foot under Rachael Blackmore to win the valuable Killashee Handicap Hurdle.
The Willie Mullins-trained mare shot clear turning into the straight and despite slowing up heading to the last, soon regained momentum to beat Joey Sasa.
Mullins said: "She's a good mare and Rachael was saying the ground made a big difference.
"She was a very very well bought mare. She was very cheap.
"Rachael is a good jockey, she's not a lady jockey, she's a good jockey."