The Newmarket handler had seen Sea Of Class go down by a rapidly-diminishing neck to Enable in the main event a couple of hours earlier.
Following a Group Three victory at Tipperary on her latest assignment, One Master was a 25-1 shot stepped up to the highest level and was nearer last than first rounding the home turn.
However, angled out by Pierre-Charles Boudot, One Master fairly flew home and got up to beat Inns Of Court and Dutch Connection into second and third respectively.
Gustav Klimt was fourth for Aidan O'Brien.
Haggas said: "It's now a good day. She's a good filly, very brave and likes soft ground. It was rough enough for her today.
"I'm thrilled to bits."