Gifted Master pipped Justanotherbottle in a photo finish to claim the Stewards' Cup at Goodwood.
The 20-1 shot, ridden by Jason Watson, trailed in the closing stages but edged ahead on the line, with 9-1 Growl finishing in third place.
Watson told ITV: "I was not too sure but I held on at the end. He is as tough as old boots and I was a little worried we got beaten."
Pre-race 9-2 favourite Foxtrot Lady finished sixth.
Watson said: "It is a massive win to get. This time last year I was watching the race on television and never did I think I would be here riding in it."
Trainer Hugo Palmer said the horse had "real class" and tipped it to be a "star" for his team.
He added: "It was a hell of a ride. He was very patient and he has a cool, calm head on him. He put his head down right on the line."
BBC racing correspondent Cornelius Lysaght
A dreamy result this for Jason Watson - already rated one of the next big things amongst jockeys, he lost the right to claim a five-pound reduction to the weight his mounts carry with a 50th career success in one of the great prizes of the summer, and looking very accomplished.
That he's a relative local to Goodwood, from Lewes, Sussex, and was surrounded by family and friends, only added to the special nature of the day. To defy top-weight in a big handicap race as Gifted Master did is quite an achievement, but he's been proving himself a quality horse on and off for a year or two.