Mexican jockey Espinoza is bidding to steer the horse to victory 12 months after guiding California Chrome to a memorable win in the one and one quarter mile gallop, the opening leg of racing's Triple Crown.
Espinoza believes he has been given a stellar ride as he aims to join the select band of jockeys to win the Kentucky Derby three times.
"I have had all the confidence in him since the first time I rode him at Del Mar," Espinoza said of American Pharoah, referring to the horse's emphatic win in last September's Del Mar Futurity Group 1 race in California.
"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," said Espinoza, who is undefeated on the horse after four rides.
"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," added Espinoza, who is unfazed by a post position of 18 in the 20-horse field.
The biggest challenge to American Pharoah's chances could come from another Baffert-trained horse, the 3-1 shot Dortmund who will be ridden by Martin Garcia out of the number eight gate.
Dortmund also arrives in Kentucky brimming with confidence following a four-and-a-half length win in the Santa Anita Derby in California in April.
The horse, owned by Kaleem Shah, has recorded a 100 percent record in six career races so far and could well make it seven on Saturday.
"It takes a horse with a certain demeanor, 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, which will be ridden by John Velazquez from post two.
Another Pletcher-trained horse, the 12-1 shot Materiality, could also upset the odds.
Elsewhere, legendary jockey Gary Stevens will be the oldest to saddle up when he rides on Firing Line, who starts at 12-1.
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 race horse, that is what one is supposed to look like," Stevens said this week.
"He's got great stamina, he's got great speed, and he's got a great mind."
The Triple Crown is completed by the Preakness at Pimlico on May 16 and the Belmont Stakes on June 6.
Only 11 horses in history have landed all three races in the same season, with the last to win it being Affirmed in 1978.