Irish trainer Aidan O'Brien sent out a rare 20-1 winner when Reply belied his odds to win the Weatherbys Insurance two-year-old Stakes at Doncaster.
The trainer's son Joseph, 18, rode the Coolmore-owned colt as he stormed to a narrow success over Factory Time (33-1) and 66-1 shot Letsgoroundagain.
Also at the St Leger Festival, Frankie Dettori enjoyed a convincing win by three-and-a-half lengths on Meeznah.
Improving middle distance filly Set to Music pushed but finished second.
After his win O'Brien junior, the oldest of four children, seemed resigned to the likelihood his days riding at the big Flat racing festivals, like Doncaster, may be numbered.
Standing at 5ft 11ins and needing to watch his diet, he admitted that his future probably lies in races staged over jumps, with their higher weight ranges.
"I can do nine stone now without killing myself," he said. "But I am tall, and I'm not getting any lighter. I'll ride over jumps and I'd like to."
Although Reply represented only Joseph O'Brien's second ever winner in Britain, he has been much more prolific at home in Ireland, and is reigning champion apprentice.
In May, he took the Irish 2000 Guineas on Roderic O'Connor, one of a number of profile mounts he's been handed by father Aidan.
And on Saturday, the teenager partners the stable's Seville (9-1) as one of nine runners due to line up in Doncaster's St Leger.
Asked about Seville, who's gained an unenviable number of 2nds in his form figures, the jockey told BBC 5 live: "He's very genuine and has been unlucky to come up against a couple of very good horses. He's in very good form."
Meanwhile Dettori said of Meeznah: "She was a bit flat at Newbury but ran a track record time at Goodwood before that, so it took a lot out of her.
"Today she was always travelling like a winner and I never saw anybody else."
Trainer David Lanigan admitted he was apprehensive ahead of the Group Two race following the disappointing result at Newbury.
Lanigan said: "Frankie was pretty sure the ground at Newbury was loose on top and they finished strung out like two-mile chasers.
"I was worried coming into it but she got her ground today and showed what a nice filly she is.
"She could run on Champions Day, or in France, and there's also the Melbourne Cup - the trip would be no problem as she stays all day."
In the day's other feature, there was a win in the Group Three Sceptre Stakes for Alanza, ridden by Johnny Murtagh and trained by Irishman John Oxx.