Gun Runner defied a wide draw to win the Pegasus World Cup Invitational Stakes at Gulfstream Park.
It had already been decided that the Breeders' Cup Classic winner would head off to stud whatever happened in the $16 million event but he signed off in style.
Jamie Osborne's Toast Of New York became the first British contender in the second running of the race but after sitting in mid-division in the first half of the contest he gradually lost touch and was allowed to coast home in the hands of Frankie Dettori.
Gun Runner was drawn outside Toast Of New York in stall 10 but he broke like a sprinter under Florent Geroux and was able to sit second on the first bend.
After that everything went to plan as Gun Runner, already Horse of the Year, followed Arrogate into the record books as a winner of the richest race in horse racing.
West Coast got closest to him with Gunnevera third but in truth it was all about one horse.
Asmussen told NBC: "It was emotional to come here for his last race given everything he's done for us. "To overcome the draw - I'm just so proud of the horse and what a dream come true he is. He's improved with every opportunity he's got, even when he didn't have success he moved forward from it. We're blessed to be in his presence. This memory will be with us forever."
Osborne tweeted: "Not our day but thankfully he seems fine. We will dust ourselves down and come up with another plan. Keep the faith! #toast."
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