Unbeaten mare Zenyatta will take on the males on Saturday in the $US5 million ($A5.54 million) Breeders' Cup Classic and was installed as the early 5-2 favourite after drawing barrier four on Tuesday.

"It's her race and she deserves the chance to show what she can do," owner Jerry Moss said of the decision to put Zenyatta in the Classic rather than the Ladies Classic, which she won last year on the same Santa Anita synthetic track.

Zenyatta is undefeated in 13 career starts but has never raced against colts.

The Classic, the highlight of the two-day, 14-race Breeders' Cup program worth $US25.5 million ($A28.23 million), will also mark the first time she has ever raced over 1-1/4 miles (2000m).

The five-year-old, trained by John Shirreffs, has won all four of her starts this year against female competition.

Shirreffs insisted on Tuesday that opting for the Classic didn't ratchet up the pressure on Zenyatta whose latest victory, in the Lady's Secret Stakes at Santa Anita, saw her equal Personal Ensign's record of 13 straight starts without a defeat.

"Equalling Personal Ensign's record was a big achievement for Zenyatta," he said.

"Now it's like starting over."

He said he and Moss both thought that if Zenyatta was training well she should have a chance to become the first female winner of the Classic.

Bob Baffert, who saddles Richard's Kid in the Classic, welcomed the move, even if it made his job harder.

"That's pretty gutsy," Baffert said. "That's what our sport needs ... it's going to make for a great championship day."

The Classic field also includes Kentucky Derby upset winner Mine That Bird and Summer Bird who jump-started his season with a victory in the Belmont Stakes and went on to win the Travers and Jockey Club Gold Cup.

More than 30 European invaders for this year's Breeders' Cup include the Aidan O'Brien-trained Rip Van Winkle - winner of the Group One Sussex Stakes and the Queen Elizabeth II Stakes this season - in the Classic.

Rip Van Winkle will be ridden by John Murtagh and was priced at 7-2 after drawing the 10th post.

The Henry Cecil-trained Twice Over, recent winner of the English Champion Stakes, will break from the fifth post - alongside Zenyatta.

In the $US1 million ($A1.11 million) Dirt Mile (1600m), the O'Brien-trained Mastercraftsman - a four time Group One winner including the Irish 2000 Guineas - drew the inside gate and was given a morning line price of 6-5.

The 1-1/2-mile (2400m), $US3 million ($A3.32 million) Turf will see Conduit, trained by Sir Michael Stoute, vie for a repeat after triumphing last year.

The Turf field also includes John Gosden's Dar Re Mi - owned by Lady Lloyd Webber the wife of famed musical composer Lord Andrew, who is suffering from prostate cancer - and another former winner, Red Rocks.

In all 10 defending or former Breeders' Cup champions are entered.

Among them, the Freddie Head-trained Goldikova will attempt to match fellow French filly Miesque - who was ridden by Head - by repeating in the $US2 million ($A2.21 million) Mile.

Friday's program includes six races, capped by the $US2 million ($A2.21 million), 1-1/8-mile (1900m) Ladies Classic.

With Zenyatta out of the picture, Careless Jewel - winner of her last five starts - was made the early favourite after drawing the inside post in the eight-horse field. Former California-based English trainer John Gosden's Rainbow View drew the outside post.