Joseph O'Brien became the youngest trainer to saddle a Breeders' Cup winner when Iridessa stormed home in the Filly & Mare Turf at Santa Anita.
The 26-year-old is also only the second trainer to have tasted Breeders' Cup glory as a jockey, after Freddy Head, following his Turf triumph aboard St Nicholas Abbey in 2011.
Iridessa was guided to success by Wayne Lordan, who was also opening his Breeders' Cup account, and he had the Pretty Polly and Matron Stakes heroine in a decent position from the start.
Mirth set the pace and soon built up a big lead, but she was swamped in the straight and it was left to Iridessa and Vasilika to fight out the finish.
Iridessa (8-1) battled on gamely to land the spoils by a neck. Last year's winner Sistercharlie was a never-nearer three lengths back in third.
Frankie Dettori was fourth on the John Gosden-trained Fanny Logan, with Just Wonderful, trained by Aidan O'Brien, fifth.
