Stayers' Hurdle champion and hot favourite Flooring Porter was in control for most of the race and looked good to secure his 11/8 favouritism. But as Danny Mullins gave him a bit of a breather as they turned for home, 16/1 shot Sire Du Berlais took his opportunity to pounce.
The pair were together at the last, but Flooring Porter landed a little awkwardly and was pulled back on track by Mullins, losing a touch of momentum in the process.
Despite slightly checking Flooring Porter in the run for home Sire Du Berlais came home a length and three-quarters ahead. Champ kept on for third, beaten by 20 lengths, with Thomas Darby a head back in fourth.
Trainer Gordon Elliott said: "He's tough. We thought he'd run well in the Pertemps at Cheltenham (11th) but that race just didn't work out for him.
"That's great, and I think the long straight really helped him as he got into a lovely rhythm and he jumped great.
"I don't know about Punchestown, we'll enjoy today and see what happens.
"I suppose it is a bit of a surprise but that's the game we're in. I'd say the better ground helped him."
Flooring Porter's trainer Gavin Cromwell was philosophical in defeat, with next year's Festival and a Stayers' Hurdle hat-trick on the agenda.
He said: "I thought he was a bit gassy and fresh going to the start and then again in the race itself.
"He got beat, but that's racing, and he was a length and three-quarters down at the line.
"I hope we can bring him back to Cheltenham for another crack at the Stayers' next season."
Champ finished one place better than at Cheltenham, although he was beaten four lengths by Flooring Porter on that occasion.
His trainer Nicky Henderson said: "He stayed on well from a fair way back. He's entered in Ireland (Punchestown) but we'll have to see."