As a record crowd cheered, American Pharoah rallied from behind and took aim at his remaining two rivals in the stretch. The bay colt and jockey Victor Espinoza surged to the lead with a furlong to go and thundered across the finish line a length ahead in the 2015 Kentucky Derby.

"There's nothing like winning the Kentucky Derby," recalled Espinoza, a three-time winner.

"To me it's the most important thing in horse racing."

America's longest continuously held sporting event turns 150 years old this Saturday (Sunday morning, AEST). By age, it's got New York's Westminster dog show beat by two years.

The Derby has survived two world wars, the Depression and pandemics including COVID-19 in 2020, when it ran in virtual silence without the usual crowd of 150,000.

The first Saturday in May is Derby Day with all its accompanying pageantry, including fancy hats, fans dressed in their Sunday best, mint juleps served in souvenir glasses, the crowd singing My Old Kentucky Home and the hand-sewn garland of red roses for the winner.

The Derby was the second-most watched sporting event of 2023 in the United States, behind the Super Bowl.

"I've heard a lot of people say it's on their bucket list to attend and [it's] the one horse race that they watch every year," said trainer Todd Pletcher, a two-time Derby winner who has this year's early favourite in Fierceness.

The one and only chance at victory

Part of what makes the Derby unique is horses have just one chance to run in it, since only 3-year-olds are eligible.

It's also the only race in America with a 20-horse field. Since 2013, horses have had to accumulate points in qualifying races to earn their way in. Previously, money won in graded stakes races decided the field.

"It's very tough to keep them on the path to get to the Derby, get into the Derby and then, hopefully, it all goes well, and you get a good, clean trip," said trainer Brad Cox, a Louisville native who will saddle Catching Freedom, Encino and Just a Touch on Saturday.

Louisvillians take pride in their city's tradition that dates to 1875, whipping up at-home versions of such high-calorie delicacies as Benedictine dip (grated cucumber, cream cheese, sour cream and mayo), Derby pie (chocolate chips and nuts) and cocktails.

Diversity struggles plaguing the famous race

Thirteen of the 15 jockeys in the first Derby were black, including Oliver Lewis, who rode Aristides to victory in front of 10,000 cheering fans in 1875.