Headline horses Coneygree and Cue Card did not complete the course as two greys dominated proceedings.
The race changed after Coneygree was pulled up by Nico de Boinville before the 10th fence after making the early running.
Blaklion and Bristol De Mai were soon to the fore as the field set off on their second circuit.
There was all to play for when Cue Card fell five fences from home when still in contention after Daryl Jacob had sent Bristol De Mai to the front after seven out.
Blaklion put in a strong challenge, but Bristol De Mai (6-1) was not for passing and crossed the line half a length to the good. Definitly Red was another 23 lengths back in third place.
Jacob said: "He's a tough horse and he was very, very fresh - he just wanted to get on with it.
"It wasn't the best of rides from me but he won anyway."
Connection of Coneygree were left dejected by the 2015 Gold Cup winner's disappointing return.
Assistant trainer Sara Bradstock said: "He simply frightened himself at the ditch. He doesn't do that normally and after that Nico said he was just scared.
"He pulled him up because he simply wasn't jumping like he normally does and he wasn't going to win.
"He is the best jumper in the world normally so something wasn't right.
"Thankfully he seems fine, he's got a little cut but nothing serious.
"Nico said the sun was very bright and it might be that he didn't see the fence.
"We'll go back to the drawing board now and try and get schooling him on the grass. We've only been able to do it on the all-weather up to now."