Pilgrims Bay produced a late surge to win the BetBright Chase at Kempton Park.
Jockey James Best edged ahead on the Neil Mulholland-trained seven-year-old (25-1) and held on to finish half a length clear.
Last year's winner Theatre Guide (11-1) had to settle for third, with Double Shuffle (11-2) finishing second.
"It's days like this that mean so much, this is the biggest day of my racing career," said Best.
"You've no idea how much it means and how grateful I am to be given an opportunity on a day like this."
There was a slight blip in the celebrations as Best was unshipped shortly after the line.
He added: "I'd done celebrating, I was trying to pull him up. I delivered him as late as I dare, with the fact he is so quirky, and sure enough he's dropped his shoulder - you have to expect the unexpected with him."