LOUISVILLE, Ky. (AP) -John Shirreffs looked up at the majestic replica of Secretariat's U.S. postage stamp looming behind him and adjusted his cap.

Big Red and his trademark checkered blue blinkers stared right back down, a larger than life reminder of the legend Shirreffs and superstar mare Zenyatta are chasing.

The unbeaten 6-year-old has become arguably the sport's most transcendent figure since Secretariat won the Triple Crown in 1973, her 19-0 record giving the industry a much-needed shot of adrenaline.

But there is more on the line in Saturday's $5 million Breeders' Cup Classic than Zenyatta's pursuit of perfection. Her place in history also could depend on whether she can beat another talented field to the finish line one more time.

She's already one of the greatest females in the sport's history. Whether she needs to reach the winner's circle under the lights on Saturday night at Churchill Downs to enter the pantheon reserved for the likes of Secretariat, even Shirreffs doesn't know.

``Somebody else will have to make that decision,'' he said.

One more brilliant stretch run by the massive dark bay could erase any lingering doubts.

And there are doubts. All but two of her victories have come on the synthetic tracks back home in California. She's only beaten the boys once, when she roared past Gio Ponti in the final yards to become the first mare to win the Classic a year ago. Her 2010 campaign has been flawless, but her five wins this year have come against fields comprised largely of unimpressive fillies and mares.

Hall of Fame trainer Bob Baffert, who will send out Preakness champion Lookin At Lucky in the Classic, is in awe of Zenyatta and grateful for what she's done for racing. He also understands the argument against her.

``She has to win here on the dirt,'' he said. ``Last year she was phenomenal, but she did it on a synthetic track. There's always going to be that question mark.''

It's a question Zenyatta can answer one ground-swallowing stride at a time. Shirreffs believes the dirt won't be a problem. Her two largest margins of victory came in the Apple Blossom on the real stuff at Oaklawn Park.

She's looked right at home at Churchill since arriving on Tuesday with the type of fanfare normally reserved for visiting heads of state.

Her daily trips from the barn to the track for a light jog look like something out of a movie star's nightmare, with a mass of cameras swimming around her as Shirreffs gently leads the way.

Not that it bothers her. Nothing really does.

Jockey Mike Smith calls her ``the ultimate entertainer,'' and she's determined to give her fans a show. She prances. She preens. She poses.

``No one tells her to,'' Smith said. ``She just does it by herself.''

Her charisma and dominance have given horse racing a figure to rally behind. She's become a part of the national conversation during her remarkable run, making headlines with her brilliance in a sport overshadowed by drugs and tragedy in recent years.

Shirreffs claims the only thing in her system beside hay, oats and water is the occasional Guinness.

``I think she's brought the sport back and is carrying it on her back right now,'' Smith said. ``She's big enough to handle it.''

Saturday's race will almost certainly be her toughest. Zenyatta will start from the No. 8 post in the 1 1/4-mile Classic and is the solid 8-5 favorite in one of the deepest Classic fields in recent memory.

Shirreffs' competitors respect her. They don't necessarily fear her.

Trainer Al Stall Jr., who will send out 9-2 second choice Blame, calls Zenyatta ``beatable.'' Todd Pletcher, who will saddle Donn Handicap winner Quality Road, considers Zenyatta one of the greats. That doesn't mean he thinks she's the been the best horse in the world this year.

While Shirreffs argues Zenyatta deserves Horse of the Year no matter what happens in the Classic because of what she's done for the sport, Pletcher disagrees.

``This is Horse of the Year 2010, so any accomplishments you have in 2009 don't count,'' Pletcher said. ``This is about 2010 and here it is, all on the line.''

Though Shirreffs believes Zenyatta's legacy is secure regardless of the outcome, he's also aware of the unique opportunity at hand.

She is racing's biggest crossover star and an unlikely success story. Owners Jerry and Ann Moss purchased her for a relatively modest $60,000 five years ago, then allowed her to grow at her own pace.

Shirreffs didn't send his gangly pupil out to the track until she was 3, and she began building her reputation as one of racing's greatest closers one heart-stopping race at a time.

Slowly the wins piled up. Ask Shirreffs if there's ever been a time over the last three years in which he thought she'd dug herself a hole she couldn't climb out of and he just laughs.

``Plenty of times,'' he said.

Yet she comes through. Always. Shirreffs doesn't know where her tenacious drive comes from and is too humble to take the credit. He knows she's more than a star, she's a symbol of what is possible.

``She represents overcoming obstacles,'' he said.

There's one more standing in the way of perfection. Her final act may be her greatest challenge. One last mad dash to the wire would silence what few critics remain and elevate her into something more than a champion.

``I think it will stamp her,'' Hall of Fame trainer D. Wayne Lukas said. ``If she whips them twice in a row in the Classic, I would have to say you'd have to mention her with the Spectacular Bids and Secretariats.''

Secretariat owner Penny Chenery is rooting for her. So is the rest of the industry. Baffert admits he'll likely have mixed emotions if Lookin At Lucky spoils the fairy tale ending.

Not Shirreffs. His big girl has done enough.

``It's not the end of the world (if she loses),'' he said. ``Giving her the opportunity to achieve something that seems almost unattainable is more important.''