Mucho Macho Man won the $5 million Breeders' Cup Classic by a nose to make Kathy Ritvo the first female trainer to win North America's richest race on a day that began tragically with the death of a horse in the first race.

Gary Stevens capped his comeback at the age of 50 in stellar fashion, surviving a photo finish in the $US5 million ($A5.3 million) Classic after winning the $2 million Distaff with Beholder on the first day of the meeting at Santa Anita.

"I didn't know the photo was as close as it was," Stevens said.

Ritvo, who had a heart transplant in 2008, became the fifth female trainer to win a Breeders' Cup race and got to celebrate a year after seeing Mucho Macho Man finish second by a half length to Fort Larned.

"Thrilling," she said.

First, though, Ritvo had to sweat out the results.

Mucho Macho Man stretched his neck at the finish to narrowly edge Will Take Charge, trained by D Wayne Lukas.

"I was just hoping he got it," Ritvo said. "When I saw Gary shake his stick, I was confident. He ran a great race."

The 30th Breeders' Cup got off to a troubling start, with a rare disqualification in the opening race and Secret Compass euthanised because of a broken leg.

The Juvenile Fillies went from strange to tragic over several minutes. The disqualification was announced after a green screen went up in a spot approaching the final turn to shield injured Secret Compass from the crowd.

"When you lose a horse like that, it just took all the wind out of our sails," trainer Bob Baffert said. "I've never had something like that happen on a big day. We're all still in shock."

John Velazquez was injured when he came off Secret Compass and had emergency surgery to remove his spleen.

Ria Antonia finished second but was declared the winner of the race after She's a Tiger was disqualified for drifting out late.

Baffert had some consolation later when New Year's Day rallied on the rail to win the Juvenile for colts and geldings but his emotions were raw.

"Just the win is exciting, but in the back of my mind, I'm thinking about that filly," he said.

Baffert and Garcia also teamed to win the $1.5 million Sprint by a neck with Secret Circle.

Current Horse of the Year Wise Dan repeated in the $2 million Mile, winning by three-quarters of a length under Jose Lezcano, a late replacement for Velazquez.

After the early drama, 3-2 favourite Dank restored order by winning the Filly & Mare Turf under Ryan Moore who added the Breeders' Cup Turf on the Aidan O'Brien-trained Magician.

The Irish 2000 Guineas winner, Magician held off another European, The Fugue, to give O'Brien his fourth win in the race.