Marsha pipped Lady Aurelia to the post to win the Nunthorpe Stakes on day three of the Ebor Festival at York.
Lady Aurelia, the Frankie Dettori-ridden favourite, led just strides from the line but Luke Morris closed aboard Sir Mark Prescott’s 8-1 shot to win in a photo finish, with Cotai Glory third.
Battaash, expected to be Lady Aurelia’s main challenger, finished fourth.
“When I saw Frankie put his arm up, I was gutted,” said Morris. “Then someone congratulated me.”
He added: “Sir Mark is a genius. I have been with him five years now and we’ve had some great days. It’s fantastic for the owners.”
Lady Aurelia trainer Wes Ward was proud of his horse in defeat.
“She’s a true champion, these things happen. She ran a brilliant race and the other filly ran us down,” he said. “I think we got beat fair and square. She’s run a fantastic race and I’m very proud of her.”