Gun Runner has won the $6 million ($A7.84 million) Breeders' Cup Classic in impressive fashion at Del Mar, trouncing a celebrated field that included last year's winner and favourite Arrogate.

Florent Geroux piloted the four-year-old to a 2 1/4 length all-the-way win over Collected in front of a packed grandstand to win America's richest race.

"I thought he was going too fast but when I saw him pulling away, it was an unbelievable feeling," Gun Runner's owner Ron Winchell said.

Up and coming three-year-old West Coast finished third with Arrogate fifth.

Gun Runner's Hall of Fame trainer Steve Asmussen said he and Geroux wanted to stick to the same game plan that propelled the striking chestnut to five wins in his past six starts.

"We wanted him away from the gates cleanly and into a nice rhythm," Asmussen said.

"The confidence that Florent has in the horse. The way the horse has been training coming into this race, I think letting him be who he is has put him on this stage and made him undoubtedly the Horse of the Year."

The 34th running of the Classic played out in front of 37,692 horseracing enthusiasts on a sunny and windy day at the picturesque track in Southern California.

The win was sweet revenge for Gun Runner, who narrowly lost to Arrogate in a thrilling race at the Dubai World Cup in March.

After a poor start, Arrogate never threatened to get near Gun Runner or Collected, who battled down the straight in the 1-1/4-mile (2000m) race.

Arrogate had hoped to join 2000 and 2001 Classic champion Tiznow as the only other back-to-back winner of the race, but it was not to be.

"He's so much better than that," trainer Bob Baffert said.

"It's just really disappointing to see him go out like that because we know how great he is. At the same time, he has given us the biggest thrills in racing and I just feel sorry for the horse."

Jockey Mike Smith said he did not think Arrogate, who has lost three straight races at Del Mar, showed his best form on the oceanside racetrack, which is known for its tight corners and firm, fast turf.

"He certainly didn't run his race today. He ran only a C effort and still got beat by only five lengths," Smith said.