Joseph O'Brien has secured his first Royal Ascot winner as a trainer after Cox Plate victor State Of Rest led from pillar-to-post in the Prince of Wales's Stakes.

Already a Group One winner in America, Australia and France, the Starspangledbanner colt State Of Rest, ridden by Irish jockey Shane Crosse, proved his class in winning on Wednesday as a 5-1 shot.

Sent straight to the lead, the four-year-old steadily raised the tempo and had seen off the challenge of Dubai Sheema Classic winner Shahryar early in the home straight.

Ryan Moore delivered 10-11 favourite Bay Bridge with what looked like a well-timed run, but he could never quite get on terms with State Of Rest, who found plenty in front to prevail by a length.

O'Brien had won the Prince of Wales's Stakes as a jockey 10 years ago aboard former Australian champion three-year-old So You Think, trained by his serial-winning father Aidan.

"We knew coming here today that we had a live chance of winning the race and I'm just pleased that the horse has proved himself. Shane gave him a marvellous ride, he has only ridden a handful of races here. He grabbed the race by the scruff of the neck," said O'Brien.

"It is a huge day, I'm very proud of everyone. We have a huge team at home and I'm so proud of them."

In October last year, State Of Rest won the $5 million WS Cox Plate, Australia's most prestigious weight-for-age race, after surviving a protest at Moonee Valley.

A flagbearer if there ever was one, the bay State of Rest may have landed the Prix Ganay, the Cox Plate and the Saratoga Derby, but Royal Ascot represented unchartered waters for both trainer and horse.

The four-year-old had never won on British turf and O'Brien had never tasted a training victory at the Royal meeting in his successful but somehow still burgeoning career.