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.
Un De Sceaux makes all and holds off stablemate Douvan to win the Grade One Champion's Chase #PunchestownFestival https://t.co/3qP47H9Tlr pic.twitter.com/XQsOkkI6E7
— RTÉ Racing (@RTEracing) April 24, 2018
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."
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