Curlin was made the early 7-5 favorite Tuesday in a field of 12 for the $5-million Breeders' Cup Classic.
Curlin would likely earn horse of the year honors for the second consecutive year if he defends his title Saturday at Santa Anita in his first race on a synthetic track.
"The synthetic is a huge question," trainer Steve Asmussen said. "A higher percentage of horses train better over it than run well over it."
Without retired Kentucky Derby and Preakness winner Big Brown in the field, Raven's Pass, winner of the Queen Elizabeth II Stakes in his last start, was made the 6-1 second choice. Like Curlin, Raven's Pass will try a synthetic surface for the first time.
Curlin will break from the No. 9 post. "Nine is a fine draw," Asmussen said. "It'll be the perfect spot for him."
Go Between, the Pacific Classic winner, was the third choice at 8-1. Undefeated Casino Drive of Japan, Todd Pletcher-trained Fairbanks, and Duke of Marmalade and Henrythenavigator, both trained by Irishman Aidan O'Brien, were all 10-1.
This year's Breeders' Cup features an all-female five-race card Friday, capped by the $2-million Ladies' Classic, previously known as the Distaff. Zenyatta is the star of that show, with an 8-0 record that made her the 3-5 early favorite.
If Curlin falters and Zenyatta stays unbeaten against defending champion Ginger Punch (9-2) and last year's runner-up, Hystericalady (15-1), she could be the rare filly to have a shot at Horse of the Year.
"That's something we really don't want to talk about," trainer John Shirreffs said.
Midshipman and Street Hero, winners of the major 2-year-old races in California, lead the 13-horse field for the $2-million Juvenile. Trained by Bob Baffert, Midshipman drew the No. 11 post for Saturday's Juvenile, one of 10 races on that day's card. Street Hero will break from the No. 8 post. Munnings is the race's 7-2 favorite.
Midnight Lute returns to defend his title as the 7-2 second choice in the $2-million Sprint for Baffert. Street Boss is the Sprint's 3-1 favorite.
Kip Deville defends his title in the $2-million Mile on the turf for trainer Rick Dutrow, who also trains Big Brown. The colt was suddenly retired because of an injury, denying the Breeders' Cup its most intriguing matchup, between Big Brown and Curlin.
"It's kind of anticlimactic because we've been running big races all year and pointing toward this," said Michael Ivarone, Big Brown's principal partner who also co-owns Kip Deville.
Kip Deville is the 5-1 third choice and drew the No. 2 post.
25th Breeders' Cup World Championships
Oak Tree at Santa Anita Park, Arcadia, Calif. $25.5-million in purses and awards
$1M Filly & Mare Sprint, 7 furlongs, 3:35 p.m., ESPN2
$1M Juvenile Fillies Turf, 1 mile, 4:15 p.m., ESPN2
$2M Juvenile Fillies, 11/16 miles, 4:55 p.m., ESPN2
$2M Filly & Mare Turf, 11/4 miles, 5:35 p.m., ESPN2
$2M Ladies' Classic, 11/8 miles, 6:15 p.m., ESPN2
$500,000 Marathon, 11/2 miles, 1:10 p.m., Ch. 28
$1M Turf Sprint, 6.5 furlongs, 1:50 p.m., Ch. 28
$1M Dirt Mile, 1 mile, 2:30 p.m., Ch. 28
$2M Turf Mile, 1 mile, 3:15 p.m., Ch. 28
$2M Juvenile, 11/16 miles, 3:55 p.m., ESPN
$1M Juvenile Turf, 1 mile, 4:35 p.m., ESPN
$2M Sprint, 6 furlongs, 5:15 p.m., ESPN
$3M Turf, 11/2 miles, 6 p.m., ESPN
$5M Classic, 11/4 miles, 6:45 p.m., ESPN