Royal Ascot winner Sioux Nation got the better of Beckford in a cracking renewal of the Keeneland Phoenix Stakes at the Curragh.
Aidan O'Brien fielded four in his challenge against the previously unbeaten Beckford, trained by Gordon Elliott.
Sioux Nation had won the Norfolk Stakes over five furlongs last time out and was the pick of the O'Brien team by Ryan Moore.
Beckford was sent off the 15-8 favourite but with a furlong to run Sioux Nation's superior mid-race speed enabled him to go a length clear.
To his credit Beckford was reeling him in all the way to the line under Declan McDonogh, but the post came too soon for him, with Sioux Nation (2-1) holding on by half a length to give O'Brien an incredible 16th win in the Group One contest.
O'Brien said: "I think his turn of foot won it, he travels very well and quickens very well - even though he's a big horse, he's got a lot of speed.
"Every run this year he has got better and has been coming forward.
"We were delighted with him at Ascot and we'd been delighted with him since.
"We wouldn't want to run him on soft ground because he has a lot of pace.
"He's by Scat Daddy, like Caravaggio. I don't know if we'll stretch him to seven this year, if we did we could come here for the National Stakes and if we didn't he could go for the Middle Park (Newmarket).
"He's a horse to look forward to."
When asked if he could be a Breeders' Cup contender, O'Brien said: "He'd have no bother, I'd say. He's big, powerful, rangy and loves fast ground. I'd say he'll progress as he's progressed with every run this year."
Stablemate Actress was third, and O'Brien went on: "She's progressing all the time and could come back for the Moyglare.
"Every run this year she has got better and better."