US racing holds breath before one last cry of 'Go Zenyatta'

The equine First Lady, unbeaten in 19 races, carries fervent hopes and a frisson of fear as she aims for a perfect 20 on Saturday

In the warren of stable blocks on the back stretch, it is not difficult to find the one housing Zenyatta. Two cars in the beige livery of the Jefferson County Sheriff are parked alongside. The police are here to keep an eye on a reverent, excited crowd, and a mare whose unprecedented achievements already qualify her as one of the greatest achievers in Turf history. Zenyatta has won every one of her 19 races. Every time she runs, the stakes seem to get higher – and on Saturday she faces perhaps her final challenge, one of the most momentous ever embraced by a thoroughbred.

With her immaculate record, defeat in the Breeders' Cup Classic would feel like the violation of a vestal. And as the hour approaches, the euphoria she has brought to the American sport is complicated by a contagious frisson of fear. She has largely raced against her own sex, in her home state of California, on safe, springy, synthetic tracks. Here she takes on a field of virile colts on the punishing dirt surface that stages the Kentucky Derby.

She won the Classic last year, but in her own backyard at Santa Anita. Taking her feminine, wilful streak to unladylike lengths, she provided one of the most dramatic spectacles in Breeders' Cup history. Barely deigning to pursue the herding males through the race, she suddenly took off from the far turn, and ran down champions from both sides of the Atlantic in the home stretch. Thousands of fans, waving pink Girl Power placards, went crazy in the Art Deco grandstands.

Among them was Pat Day, who had won the race four times in his riding days. Yesterday he was among the throng who followed Zenyatta back to her barn after she had breezed round the dirt circuit. Day sought out her owners, Jerry and Ann Moss, and told them: "In all my years on the racetrack I never saw anything so electrifying as the Breeders' Cup Classic last year. I was standing up on my chair, screaming and crying."

Having initially resolved to retire Zenyatta, the Mosses had a dramatic change of heart last winter, and have since gained a touching sense that their ownership nowadays is largely titular. True, she will be trying to win them the biggest prize in the American sport, but if you divided $5m (£3.1m) among all those who feel a proprietary interest, they would barely get a dime apiece.

As if to corroborate their status as mere fans, the Mosses and their trainer, John Shirreffs, were themselves taking pictures yesterday as the glossy beast made her way to and from the track under a bruised, sallow sky. The big, dark mare is always on parade, pawing and preening and strutting. When she arrived in Louisville on Tuesday, she was escorted from the airport by police cars and a helicopter. Oprah Winfrey included Zenyatta in her Power List of the most influential females of 2010. You almost feel as though the mare suffers Mike Smith to sit on her back only because his everyday name would not deceive anybody that he contributes anything to her stardom.

Yesterday Smith duly took his place among the spectators, her exercise instead entrusted to a hollow-cheeked, 67-year-old work-rider named Steve Willard, known to all on the back stretch as "Grandpa". Willard's frilly leather chaps are worn through on the inside of his calves, and something similar will be happening to his heart in the melancholy likelihood that this will be the mare's final race. "She has so much want," he says. "We've never got to the bottom of her."

Smith was, meanwhile, being embraced by Angel Cordero, a bigger star in his time even than Day. "I am so happy for you," Cordero said. "Good luck!" Few would envy Smith's duty on Saturday, when charged with not screwing up by so many. Zenyatta always tests their nerves by leaving things late, and has won her last three starts by no more than half a length. "It's almost as if she doesn't want the other horses to feel bad," Ann Moss says.

She smiled to herself as Zenyatta cooled off, strolling round the barn. "She needs a bigger blanket, don't you think?" she said. "She makes it look like a bikini." The Mosses remain a striking pair. She was once a model and her husband, at 75, stands tall under a mane of grey hair, his aquiline features frequently breaking into a benign grin. He made his money from a record label that stabled some of the top artists of the 1960s, 1970s and 1980s, including Sting, whose Police album Zenyatta Mondatta inspired the mare's christening.

Nor will you find anyone round here to begrudge Shirreffs his good fortune. After serving with the marines in Vietnam, he just wanted to surf, to become a beach bum. He was heading to the Pacific, riding across a Californian valley, when his horse sank over his hocks into a bog. Shirreffs extricated the animal, little realising his efforts were being admired by a ranch manager, who promptly offered him a job. So began an odyssey that seemed to reach its consummation when Shirreffs came here in 2005 and won the Derby with a 50-1 chance named Giacomo. At the time he impressed everybody with his easy, reflective demeanour, but none could imagine he would return at 65 with a shot at greater glory still.

True, there has been a mean-spirited reluctance in some American professionals to salute the mare. Last year, Horse of the Year voters instead favoured Rachel Alexandra, whose connections had refused to run her on what they disparaged as a "plastic" surface at Santa Anita. The introduction of synthetic surfaces has created a bitter schism in the American industry. Zenyatta's arrival here, at the spiritual home of dirt, is duly saturated with symbolism – and some would discover a deep, secret satisfaction in seeing her beaten at last under Churchill Downs' twin spires.

The majority, however, can perceive a messianic quality in Zenyatta. Sooner or later, anyone who works with thoroughbreds is made to see them as agents of a random fortune. Even the best will sometimes get beaten – mutely nursing some latent discomfort, perhaps, or held up in traffic, finishing too late. Given her running style, it's amazing that Zenyatta has never done that. In her metronomic brilliance, she seems to warrant the hopeless, Sisyphean rituals of horsemen everywhere. There are hundreds of other horses stabled here, and they all get the same treatment. Every morning, the same modest ceremonies are patiently played out – the splashing of hoses, the pitching of steaming straw. Here is a creature to give hope to everyone on the racetrack. Here is one dream, so it seems, that was not doomed to drain away with the soapy water, to putrefy in the muck heap.

They are billing Saturday's race as "the quest for perfection". Such is the folly of men; such is the disappointment they guarantee in horses. This time, though, they reckon to have found one equal to their most ingenuous fantasies. These will be two of the most pulsating minutes in American sporting history. The sense of benediction is not lost on Ann Moss. "Zenyatta touches people's hearts," she said. "It's amazing to see people jumping up and down, hugging people they don't even know. She's a joy-maker. Every day with her is a celebration."

Turf account

Chris McGrath's Nap

Dubburg (7.30 Kempton)

Down to a lenient mark and for the third time running shaped as though ready to take advantage over course and distance last time, set plenty to do and hampered on his way through into fifth.

Next best

Rule Of Nature (1.20 Lingfield)

Has repeatedly promised to exceed this kind of rating but lost her way before shaping much better on her last two starts, left too much to do when hampered last time but still managing fourth. Extra furlong a help here.

One to watch

Joviality (J H M Gosden) made a very encouraging debut at Newmarket last weekend, working her way past all bar two in a big field while seeing plenty of daylight and running green.

Where the money's going

Zenyatta is 2-1 from 5-2 with Coral to crown her unbeaten career in the Breeders' Cup Classic on Saturday.