The first qualifier for Sunday's prestigious Rolex Grand Prix at CHIO Aachen, the Turkish Airlines-Prize, was taken by the US rider.
Friday's win aboard Contagious puts the American at the top of the leaderboard going into Sunday’s World Cup title decider.
The US rider and his Olympic mount edged Belgium's Zoe Conter riding Davidoff de Lassus in the 46-entry field.
Round 4 of this series attracted 65 entries to compete over Anderson Lima's course on the Derby Field, 12 of which came back for the jump-off.
The pair had the only double-clear effort to clinch the victory in Lexington; Tiffany Foster and Figor top Canadians in 4th.
The Israeli rider edged McLain Ward and Contagious for the victory; Canadian Mario Deslauriers rode Uris de la Roque to 4th place.
Mario Deslauriers is top Canadian, riding Uris De La Roque to 6th place in the Saturday afternoon class on the Derby Field at WEF.
US rider McLain Ward piloted the gelding to the win in a massive field of 100 entries, while Canadians earned a couple of top-10 spots in feature classes.
McLain Ward has withdrawn Contagious for the Lima 2019 Pan American Games due to injury. The pair will be replaced by Lucy Deslauriers and Hester.