The world's best horse Arrogate has looked anything but, running a distant fourth in the Group Two San Diego Stakes won by Accelerate at Del Mar.

Jockey Mike Smith tried everything he could with Arrogate but nothing worked.

"He just wasn't trying," Smith said.

Accelerate beat Donworth by 8-1/2 lengths with Arrogate 15-1/4 lengths from the winner.

"I'm at a loss for words," Smith said. "He was just flat, so flat."

His trainer, Bob Baffert, was blunter, saying Arrogate "laid an egg."

"I thought he'd run better than that," Baffert said. "But he just didn't want to go."

Ridden by Victor Espinoza, Accelerate lived up to his name, leading all the way.

"He got in a nice rhythm. He was going easy," Espinoza said.

"I was surprised by how far I won. I wasn't surprised that I did. I was here to win."

All but $214,466 of the $2,671,938 total wagering pool was bet on Arrogate, the 1-20 favourite.

Arrogate's seven-race winning streak until Saturday (Sunday AEST) included the $6 million Breeders' Cup Classic last year, the $12 million Pegasus World Cup and the $10 million Dubai World Cup.

It wasn't the first time Baffert had led a world-beater to the track only to watch him lose.

That's what happened in 2015, when American Pharoah was stunned by Keen Ice in the Travers barely two months after becoming horse racing's first Triple Crown winner in 37 years.

"That's why my hair is white," Baffert said.

With Accelerate setting the pace, Smith had Arrogate last through the opening half-mile before they began making their move into the far turn. But Arrogate's rally was brief while Accelerate cruised to the finish line in front of a shocked crowd at the seaside oval north of San Diego.

Arrogate will have a chance to turn the tables in the $1 million Pacific Classic on August 19.