Gust Of Wind, the last horse to beat Winx, has been retired after a suffering a heart fibrillation during the Tramway Stakes.
The John Sargent-trained mare beat Winx to win the 2015 Australian Oaks and finished a solid fourth in last year's Caulfield Cup.
She was due to be served by Criterion next week with a view to another Cups campaign before she became a mother.
But her owner Sir Owen Glenn immediately decided her racing career was over and she will stay in the broodmare paddock after she visits Criterion.
Gust Of Wind finished a long last in the Tramway and Sargent said it was obvious something was wrong so the decision to retire her was not difficult.
Since her second to Gust Of Wind, Winx has won 10 races including the Cox Plate and Doncaster Mile.