Triple Crown winner American Pharoah has cemented himself as a true champion with his win in the Breeders' Cup Classic.

In what was the three-year-old's swan song before he heads to stud, American Pharoah took charge from the outset, winning the $US5 million ($A7.07 million) by 6-1/2 lengths on Saturday (Sunday AEDT).

The colt smashed the 1-1/4 mile (2000m) track record by more than five seconds, running 2min00.07s as the sentimental 3-5 favourite among the crowd of 50,155 at Keeneland.

"This was for Pharoah," trainer Bob Baffert said. "We wanted him to go out the champion he is."

Effinex was second with Honor Code another 4-1/2 lengths third in the eight-horse field.

It was a feel-good moment for a sport that has been battered and bruised - all the troubles of declining attendance and drug controversies were wiped away in two magical minutes.

"It's a horse racing fairy tale and I just happen to be in it," Baffert said.

After easing across the finish line, Espinoza took the colt far up the first turn before slowly walking past the grandstand to the winner's circle, accompanied by raucous cheers all the way. The champion even had his own military escort walk him back to his barn.

The fans knew they had just witnessed history, the final chapter in a story that may never be repeated.

American Pharoah put an exclamation point on a brilliant career in which he lost just twice - in his debut and again in the Travers on August 29.

"It's a lot of pressure to train a horse like this because I didn't want to let the horse down and I didn't want to let the fans down," Baffert said.

"I'm just so proud of him. It's like watching my child out there."

American Pharoah won nine of his 11 career starts, including the first sweep of the Kentucky Derby, Preakness and Belmont in 37 years.

He earned a total of $8,650,300 for Ahmed Zayat, his Egyptian-born owner.

"We wanted him to go out as a winner," Zayat said. "He is a winner."

Next up for American Pharoah is a new career as a breeding stallion at Coolmore's Ashford Stud near Keeneland.