Peeress was a surprise 14-1 winner of the Windsor Forest Stakes, getting up late under Mick Kinane.
Sundrop, the 15-8 favourite, made most of the running and went for home with three furlongs to run, but she began to flag inside the final furlong and was collared by Peeress, who had been prominent throughout.
She kept on to score by two lengths from Sundrop, with Soviet Song third.
It was a first winner at this season’s Royal meeting for Kinane and trainer Sir Michael Stoute.
Soviet Song raced in third for most of the way and Johnny Murtagh had to nudge her along before many of her rivals, but it was still a respectable performance for her first run of the season.
Peeress was something of a super-sub for her connections’ other star Chic, who was withdrawn yesterday.
She was also reversing Epsom form with Sundrop, who defeated her by two lengths in the Princess Elizabeth Stakes on Derby day.