"We had a Triple Crown victor three years ago", he said, "and that was the first time in 37 years (since Affirmed in 1978), so that sort of took some of the luster off of Justify's achievement-not a lot, just a little bit". "He's won all those great races".
"Accelerate ran against all ages, but for me it's a real tossup".
"When I got to the backside I tried to be in the clear, which is what he likes to do".
O'Brien, who has won 12 Breeders' Cup races but never a Classic, said Mendelssohn has "toughened up" since he broke poorly and was pulled up to finish last out of 20 in the Kentucky Derby in May. Accelerate has won five Grade I races in this year alone.
Gunnevera finished in second behind Accelerate in Saturday night's Classic, with Thunder Snow rounding out the top three.
"They looked good for a mile", Baffert said.
Hall of Fame trainer Bob Baffert, the man behind Triple Crown winners American Pharoah and Justify, is targeting a fourth Classic win in five years. "They just exhausted and just didn't have it today".
Mendelssohn, a familiar name from the Triple Crown circuit earlier in the year, rounded the turn ahead of the pack in what's always been deemed the Super Bowl of horse racing. And it followed other notable performances on the second day of the $30 million championships.
The John Gosden-trained filly didn't get her four-year-old season underway until September thanks to a leg injury, but that didn't stop the bay star from winning a second straight Arc on October 7 with Frankie Dettori in the irons. It was the fifth win in 10 starts for the Sir Michael Stoute-trained colt.
The one ray of hope for the competition is the statistic that Arc winners are 0-7 when taking on the Breeders' Cup Turf in the same year.
Enable made history at Churchill Downs in Saturday's US$4m Grade 1 Breeders' Cup Turf (1m4f).
"These horses, the field, they've made over $36 million - that's remarkable", said United States trainer Bob Baffert - who saddled Classic winners in 2014, "15 and '16 but has never won the biggest Breeders" Cup prize when the two-day, 14-race festival has been held beneath Churchill's twin spires in Louisville, Kentucky.
She recorded back-to-back Arc victories last month and was strongly fancied in her bid for immortality.
He had to wait for a review before wearing the purple and yellow Breeders' Cup winner's blanket as the stewards looked at a bump with Uncle Benny just before the wire. Not only was the start of her 2018 campaign delayed by injury, Gosden said there was the "slight hiccup" of a fever between her first start this year, in the September Stakes at Kempton, and the Arc.
The Pennsylvania bred recently won the La Lorgnette Stakes at Woodbine and past year, opened her career with a win in the Ontario Debutante Stakes and a second-place finish in the Mazarine Stakes.