The 11-2 shot Rizeena recovered from a disappointing run in the 1,000 Guineas at Newmarket to win the Coronation Stakes at Royal Ascot.
The winner of the Queen Mary Stakes at last year's meeting was well back at Newmarket and then missed the Irish Guineas after breaking a blood vessel.
But the horse, trained by veteran Clive Brittain and ridden by Ryan Moore, showed she was back to her best.
She beat Lesstalk In Paris (15-2) with Euro Charline (16-1) third.
Moore was celebrating his third winner of this year's Royal meeting after success on Integral (Duke of Cambridge Stakes) and Cannock Stakes (Tercentenary Stakes).
"She was the best filly last year. She won the Queen Mary. It's good for her to come back and I'm delighted for everyone," he said.
There was drama at the start of the race when Kieren Fallon was unseated on leaving the stalls from the American challenger Rosalind and none of the remaining 11 runners were keen to set the pace.
Eventually Jimmy Fortune sent J Wonder to the head of the field but two furlongs from home, Moore made the decisive move and the three-year-old kicked clear to win by three-quarters of a length.
Moore added: "There was no pace, it was a crawl, it was ridiculous really. We went quicker to the start than we did for the first quarter-mile.
"She doesn't want to be in front that early, but my hand was kind of forced. It was a long way that last furlong, but she was the fastest horse in the race and she's just done enough."
It was an 18th Royal Ascot winner for Brittain, 80, whose first winner at the meeting was 40 years ago.
"I wasn't worried about the slow pace" he said. "They could have walked - she's a great walker - and still would have won, but they galloped and she still won. We covered every angle.
"I've still got the vibes. If you get up at 3.30am every morning you've got to have the vibes."