The Queen has publicly thanked well-wishers who sent her messages of support after her horse triumphed in the Gold Cup.
As Royal Ascot began the Queen highlighted the "wonderful welcome" the filly received last year after romping home in one of the few prestigious races to have eluded the monarch.
Prince Harry officially joined his grandmother for the first time at the famous Berkshire meet, a mainstay of the racing calendar the Queen described as "one of Britain's finest sporting events".
Her thoroughbred Estimate won the Gold Cup at Royal Ascot last year and she could not contain her delight when it passed the post first.
Writing the foreword for the official programme the Queen said: "I would like to thank those of you who were here for the wonderful welcome the filly received after performing so well, and also those who were kind enough to send messages afterwards.
"To win at Royal Ascot is challenging and I know how much work goes into preparing every one of the many horses racing during the week. All those involved deserve great credit, as well as our gratitude."
The Queen is a passionate horse owner and breeder and during her long association with Royal Ascot has had 22 winners.
Estimate will attempt to repeat the success of last year when it runs in the Gold Cup on Thursday.
The world-famous annual meet is as much a social event as an important sporting fixture and it attracts some of the best thoroughbreds, jockeys and trainers in the world.
The Queen started off festivities by taking part in the traditional carriage procession along the course.
She was joined by the Duke of Edinburgh, Duke of York and Harry in her carriage and as they passed the stands a cheer went up from the punters.
Following in another carriage was the Prince of Wales, the Duchess of Cornwall, who has recovered from a bout of sinusitis, and Princesses Beatrice and Eugenie.
Other royals who attended included the Duke and Duchess of Gloucester and Prince and Princess Michael of Kent.
In the final carriage was the Queen's nephew Viscount Linley and his wife Serena, they sat opposite Viscount Ullswater, and his wife Susan, who was private secretary to Viscount Linley's mother Princess Margaret. The Princess Royal had arrived ahead of the others by car with her daughter Zara Phillips.