Japan narrowly outpointed his better-fancied stable companion Mount Everest to land the Group Two spoils in the Beresford Stakes at Naas.
Mount Everest looked like justifying 11/8 favouritism when he took over from Sovereign, Aidan O'Brien's third runner in the mile heat, but could not quite shake off the opposition.
Seamie Heffernan got a terrific response from Japan (11/2), who hit the front in the final few strides to get the verdict by a short head from Ryan Moore' mount.
The leading pair pulled three lengths clear of Michael O'Callaghan's Power Of Now in third, with Sovereign fourth.
It was O'Brien's eighth successive win in the Beresford and took his record tally to an amazing 18.
"He was very green the first day and won nicely the next day. We thought he would come forward again from this and he probably will," said O'Brien.
"You couldn't be happier. He followed the pace when the other two were going along.
"I thought he'd come forward again from today, he's a big, hardy horse, so he could be the type of horse for what was the Racing Post (Vertem Futurity Trophy at Doncaster).
"He travelled well and did everything right, so you'd have to be delighted with him really."
O'Brien added: "The second (Mount Everest) did nothing wrong and ran a lovely race, too. There were no prisoners taken and it was an honest straightforward race.
"We're all learning together. Donnacha's (O'Brien, Sovereign) is a massive big horse and they are all big middle-distance colts. The experience will do them really good."
O'Brien was completing a 306/1 four-timer thanks to the earlier victories of Magna Grecia (2/1 favourite), Hermosa (5/4 favourite) and Cypress Creek (6/1), who were all ridden by Moore.
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