Victor Espinoza believes Kentucky Derby favourite American Pharoah is untapped as the jockey bids for back-to-back wins in the famous race.

Espinoza, who guided California Chrome to victory a year ago, said barrier 18 was not a problem for the Bob Baffert-trained colt in the $US2 million ($A2.53 million) race on Saturday (Sunday AEST) at Churchill Downs.

Last year's champion US two-year-old, American Pharoah is at 5-2 to give Espinoza another victory in the Derby (2000m) which he also won in 2002 on War Emblem.

"I have had all the confidence in him since the first time I rode him at Del Mar," Espinoza said.

"The way he won that day, I just said 'Wow, he's an amazing horse.'"

Recent results have underscored American Pharoah's status as the horse to beat, with a victory in the Rebel Stakes at Oaklawn Park in March followed by an impressive eight length win in the Arkansas Derby at Oaklawn in early April.

Ominously for his Kentucky Derby rivals, Espinoza believes American Pharoah's full potential may yet to be realised.

"I really don't know how good he is," Espinoza said.

"I ride him, but the way he does things so easy, I really don't know how good he is. He's an amazing horse. But so far, I never ride him really hard."

The biggest challenge to American Pharoah's chances could come from another Baffert-trained horse, the 3-1 shot Dortmund to be ridden by Martin Garcia.

The unbeaten Kaleem Shah-owned Dortmund arrives in Kentucky following a 4-1/2 length win in the Santa Anita Derby in California in April.

"It takes a horse with a certain demeanour, and speed and class to be undefeated, 6-for-6 so far," Shah said.

"I don't want to get ahead of myself, but I'm very hopeful that he will do well for himself."

The 8-1 third pick is Carpe Diem, one of three horses in the Derby trained by Todd Pletcher and to be ridden by John Velazquez.

Legendary jockey Gary Stevens will be the oldest to mount up when he rides 12-1 chance Firing Line.

The 52-year-old, a three-time Kentucky Derby winner, is impressed with what he has seen from Firing Line so far.

"If you want to picture a racehorse, that is what one is suppose to look like," Stevens said.

"He's got great stamina, he's got great speed, and he's got a great mind."

The Kentucky Derby is the first leg of the Triple Crown which is completed by the Preakness at Pimlico on May 16 and the Belmont Stakes on June 6.

Only 11 horses have landed all three races with Affirmed the last to do so in 1978.

California Chrome won the first two legs last year.