Lady Livius, ridden by Martin Dwyer, came home at a huge price to win the Weatherby's Super Sprint at Newbury.
The 100-1 chance stuck to the much less-fancied far rail but she still showed too much speed for the rest of the field.
Nigella, ridden by Fran Ferris, finished fast but just failed to overhaul the winner late on.
Lady Livius was one of Richard Hannon's five runners and gave the trainer his sixth success in the race.
Hannon said afterwards that was as surprised as anyone to see Lady Livius prevail against her two-year-old rivals.
"I couldn't see her winning it from stall one but If Paradise won from that
side as well," he said about his 2003 winner.
"I haven't got a clue where we'll go now. I kept telling the owner John Lee not to bother running her here and to wait for a maiden at Windsor next week.
"But he said that he'd paid the money to enter her and he wanted to see her
run."
Weatherbys Super Sprint (5f)
1. Lady Livius (Martin Dwyer) 100-1
2. Nigella (FP Ferris) 16-1
3. Brandywell Boy (T Quinn)
25 ran