The Colin Tizzard-trained Native River hung on to win the 60th running of the Hennessy Gold Cup at Newbury.
The 7-2 favourite crossed the line half-a-length in front of fast-finishing Carole’s Destrier (25-1) with Double Ross (50-1) a gallant third.
It was a first victory in the race for champion jockey Richard Johnson, who had finished second on two occasions.
Earlier, Tizzard’s Cheltenham Gold Cup favourite Thistlecrack cruised to victory in the bet365 Novices’ Chase.
Tizzard is enjoying a golden spell having saddled Cue Card to win last week’s Betfair Chase at Haydock.
Native River appeared to be coasting to a comfortable win in Saturday’s big race before Carole’s Destrier gave him a real fright.
Tizzard said: “It looked like he would win by five lengths, but when Carole’s Destrier nearly got to him, he went again. It is a sign of a good, honest stayer.
“We expected a big run, but he travelled better than ever before and jumped beautifully.”
The trainer later claimed a treble on the card when Ultragold (9-2) won the bet365 Handicap Chase.
Cornelius Lysaght, BBC horse racing correspondent
Another Saturday, more big-race success for the Tizzards – trainer Colin, wife Pauline, ex-jockey-son Joe and daughter Kim – who make up the very much in-form family-run operation based in Dorset.
On one of the great days of the year, Thistlecrack was flawless, Native Run landed a huge gamble after battling back from losing his long-time lead and then Ultragold got up close to the finish to end a heady day.
Once it was doubted that Native River was quite good enough to join Cue Card and Thistlecrack in the Cheltenham Gold Cup, but after this thoroughly likeable performance, achieved in a decent time, that no longer looks the case.
The one-time dairy farmers three-handed-PLUS in the Gold Cup? More than possible, and what a compelling story for the sport between now and March.