The unbeaten Shared Belief is the early favourite for the Breeders' Cup Classic ahead of Kentucky Derby winner California Chrome.

Shared Belief is at 9-5 after drawing gate six with California Chrome (4-1) to jump from 13.

Mike Smith, who has won more Breeders' Cup races than any other jockey, will be aboard Shared Belief in the $US5 million ($A5.41 million) Classic (2000m).

"He's got speed all around him and we'll be able to follow them into the first turn," Smith said.

California Chrome's trainer Art Sherman is not concerned about the barriers but one of the colt's owners has said he will be retired if he runs poorly on Saturday after two recent defeats.

"I just hope he has a great race," Art Sherman said. "He loves Santa Anita. The horse is training great and I look for a big performance out of him. I love the post position. I don't have to be down there (inside)."

Tonalist, who ended California Chrome's Triple Crown bid with a victory in the Belmont, drew No.11 and is the 6-1 third choice.

Untapable, the nation's top three-year-old filly, is the 5-2 favourite for the $US2 million Distaff, one of four Breeders' Cup races on Friday.

American Pharoah drew post position five in a field of 13 for the $US2 million Juvenile, where the two-year-old will try to position himself as the early favourite for next year's Kentucky Derby.

American Pharoah will be ridden by Victor Espinoza, who rides California Chrome in the Classic.

The Juvenile, to be run Saturday, includes two entries each from Bob Baffert, Todd Pletcher and Irish trainer Aidan O'Brien.

An ankle injury has kept two-time Horse of the Year Wise Dan out of the $US2 million Mile, and his defection triggered a flood of 25 nominations. The maximum field of 14 includes Obviously, who finished third and fifth in the past two runnings.

There is $US26 million on offer over the two days of racing.