"I just thought I had to go ride it well," the 22-year-old said.
"We just had to keep the rhythm and keep him going - Biarritz was just outstanding."
The 12-year-old Selle Francais, who is by Lindberg des Hayettes, was certainly very excited coming into the tight indoor arena.
"He is very alert and aware of what is going on around him so does get a little overexcited," said Morrison. "He is just super though. He tried his heart out. I am so happy."
She felt when she walked the track, most competitors would have been fine, however once in the ring, realised it took a lot of jumping.
"The lines were forward, so you had to land and keep going. When I walked the track with Helen (McNaught-McFarlane) and Duncan (McFarlane), they just said to not panic. I owe a lot to them - they have been phenomenal this season with their support and teaching."
Next on Morrison's list is Equitana Auckland, followed by the remainder of the World Cup series and then Horse of the Year.
"I would like to win the series this year - I was third last season - and of course the Olympic Cup is a big goal."
Sunday's win moved Morrison up to second on the World Cup leaderboard, with Afleld holding on to her lead by just two points.
The next qualifier is in Hawera later this month.