Mucho Macho Man has won a Breeders' Cup Classic thriller, edging Will Take Charge in a photo finish to give trainer Kathy Ritvo an emotional victory.

Ritvo, a heart transplant survivor, became the first woman to train a winner of the North America's richest race when Mucho Macho Man erased the memory of his narrow second to Fort Larned last year.

Veteran Gary Stevens, riding again this year at the age of 50 after seven years in retirement, piloted the five-year-old to the victory in the $US5 million ($A5.30 million) Classic.

The Aidan O'Brien-trained Irish horse Declaration of War finished third while favourite Game On Dude was ninth.

Coming out of the final turn, Mucho Macho Man took the lead with Declaration of War on his inside along with Fort Larned. Game On Dude, ridden by Mike Smith, dropped out of contention entering the straight.

"I pushed the button," said Stevens, who had never before won the Classic.

Stevens celebrated when he crossed the line, but it took a review of the photo-finish to award Mucho Macho Man the victory by a nose.

"I heard 'em coming," Stevens said. "I knew they were coming. But he was super, super tough."

Stevens admitted if he had realised just how close it was, he might have held off on the celebration.

"But the race was a mile and a quarter, not a mile and a quarter and a stride," he said.

Ritvo said she was sure that Mucho Macho Man had done it when she saw Stevens celebrate.

"When I saw Gary shake his stick, I was confident," she said.

Fort Larned held on to take fourth.