American Pharoah has been put to the test and come out on top in the Kentucky Derby at Churchill Downs.

Sent off the 5-2 favourite in front of a record crowd of 170,513 on Saturday (Sunday AEST), the brown colt rallied in the straight to beat Firing Line by a length to deliver trainer Bob Baffert his fourth Derby win.

"It's a fantasy moment for us," Baffert, who also saddled third placed Dortmund, said.

"We were ready to rumble."

Dortmund set a leisurely pace with Firing Line tracking him closely in second. American Pharoah sat comfortably in third until the home turn.

That trio made it a three-horse race in the straight, with none of the closers able to make up ground. American Pharoah angled outside and fought off a persistent Firing Line as Dortmund tired along the rail.

Victor Espinoza won his second consecutive Derby a year after being aboard California Chrome, and third overall.

"He's been a special horse since I first rode him," Espinoza said. "I feel like the luckiest Mexican on Earth."

Owner Ahmed Zayat accepted the gold winner's trophy - his first after a trio of second-place finishes.

American Pharoah missed his first big test last year when he was scratched from the Breeders' Cup with an injury.

He returned with two easy wins this year against lesser competition. Dortmund and several other Derby contenders had beaten much tougher fields, raising questions about whether American Pharoah could mix it up in a 20-horse field.