Kerins and mount E Muze Yek rode a clear round, and then were fastest in the jump-off for Kerins' first victory in the International Ring.
Nine of the 28 entries had a clear round and advanced to the jump-off. Kerins and E Muze Yek went second and put up a time of 40.165 seconds that could not be bested.
Ashlee Bond of the United States finished second, while Ian Millar of Perth, Ont., was third riding Star Power to a jump-off time of 41.023 seconds.
Also Thursday, American McLain Ward and Zander rode to victory in the ATCO Pipelines Cup, while Lauren Tisbo of the United States took the Prairie Mines & Royalty LTD Cup aboard Roundthorn Madios.
The National tournament continues through Sunday.