Deusser finished a fraction of a second ahead of Egyptian Nayel Nassar, while Swede Peder Fredricson finished third.
“I had a big hope in that horse,” Deusser said of his 10-year-old mare Jasmien v Bisschop.
“Her first big, big win makes me really proud,” he added. “I knew I had to take some risk and be really fast. It was very, very tight but I am very proud it worked. I wanted to win.”
The victory allowed Deusser to overtake Belgian Pieter Devos for first overall in the GCT ranking. The two are now separated by 10 points, after Deusser triumphed in a four-horse jump-off in Hamburg.
Devos rode solidly in the first five stops of the tour — including a win in Miami in April — but faltered in Hamburg, knocking off a top rung toward the end of his routine in Round A.
The seventh round of the 2019 Longines Global Champions Tour and its team-based counterpart, the Global Champions League is set to take place in Cannes, France from 6-8 June. The 20-stop circuit culminates in the Prague Playoffs from November 21-24.