Jockey William Buick rode the Australian horse to victory, narrowly ahead of Spirit Quartz.
Ortensia finished strongly to delight trainer Paul Messara.
"That was fabulous," said Messara. "Winning a big one like this over here was huge for us."
Buick told Channel 4: "I don't know how she won. It was very fast.
"I can hold a straight course all the way, and she's so much better when she doesn't have to twist in or out."