The race-favourite's owner, David Brotherton, chose Supple ahead of several better-known names to partner his runaway Portland Handicap winner after Kieren Fallon was claimed by Ian Balding to ride the second favourite, Grangeville.
"Willie rode Surprised for me on Friday at Doncaster and I thought Surprised ran the best race of his life," Brotherton said. "I think Willie is a very good jockey and a very good horseman."
It is the latest recognition of Supple's talents in a memorable season for the Irishman. The 31-year-old, who has won the riders' championship in Dubai in the last two winters, has ridden 35 winners in Britain this year - surpassing his previous best total of 21 set last season.
However, Supple is playing down his chances on Saturday. "He was very impressive last time, but there is no such thing as a certainty in racing."
Brotherton added: "Anything can happen in a big handicap and you need a lot of luck. I have been in racing a long time and I know what happened to certainties - they get knocked over."
The John Hammond-trained Astonished, the sponsors' 7-2 favourite, steps up from the Portland trip on Saturday, when the going is likely to be a good deal softer than he encountered at Doncaster. But Brotherton said: "The ground hopefully won't inconvenience him - he has run well on good to soft before. And he won over six furlongs at Deauville and over seven furlongs at Doncaster on Leger day last year."
Pipalong, the winner of the Great St Wilfrid Handicap, has been confirmed as a runner in Saturday's race. The Tim Easterby-trained filly has been allotted 9st 5lb in the pounds 100,000 sprint and the trainer's father, Peter, said: "She's grand and runs at Ayr but we haven't made any riding arrangements yet."
Easterby snr respects the chance of Astonished. "I'd like to swop at this stage but you never know, he could get a bad draw."
Pipalong is at her best with give in the ground and underfoot conditions appear likely to favour her on Saturday. Richard Pridham, clerk of the course at Ayr, yesterday reported: "The going is good to soft, soft in parts. It is a drying day and we could have good ground by Thursday."
Always Alight will be reunited with last year's jockey, John Egan, when he attempts an historic repeat success in the race. Karl Burke has turned back to the Irishman for his five-year-old who, if successful, would be the first multiple winner of Scotland's biggest race since World War II.
Royal Anthem is to be trained in America after his flop in the Irish Champion Stakes, on Saturday. The four-year-old, a well-beaten fifth behind Daylami when favourite at Leopardstown, is set to leave Henry Cecil to be trained by Bill Mott in California.
It is a bitter blow for Cecil, who hailed Prince Ahmed Salman's colt as one of the best he had trained after a remarkable eight-length win in last month's International Stakes at York. "It's sad he's going and everyone at Warren Place will miss him a lot," Cecil said. "He is a very, very good horse who ranks among the best I've been lucky enough to train."
Prince Ahmed's racing manager, Dick Mulhall, said: "We may think about the Breeders' Cup Classic but Gulfstream Park is a tight track and it does get hot there."
Nap: Triple Green
NB: White Emir
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