Theirs is a May-December relationship that began almost four years ago. She had spurned a handful of younger, would-be partners who tried to control her. He let her be herself.

Steve Willard is on top of the world these days, a fact he readily attributes to being atop one of the greatest racehorses in modern times. As exercise rider for the 6-year-old champion mare Zenyatta, who has only to win two more races to retire with a perfect 20-for-20 record, Willard has reached the top echelon of a generally thankless profession.

Often the first to know when a horse is ailing, exercise riders also account for the most on-track human injuries. Some of the more experienced riders are salaried; most get paid around $15 a horse. The median age of an exercise rider is 35, but a quick appraisal of the baby-faced men and women taking their charges out to the Hollywood Park racetrack this morning suggests that Willard may account for this ostensibly inflated average.

It’s a career that Willard, 67, will soon walk away from, and for that alone he is thankful. One week ago, a 24-year-old jockey named Michael Martinez was paralyzed from the waist down when a horse he was riding clipped heels and fell on him in a race at Golden Gate Fields, severing his spinal cord. Willard’s girlfriend, Kiki Spencer, was forced to quit galloping horses three years ago when a horse fell and rolled over on her, resulting in a compound fracture of the collarbone.

Willard was prepared to retire last year, along with Zenyatta, after her breathtaking win in the Breeders’ Cup Classic at Santa Anita. A&M Records founder Jerry Moss, who owns Zenyatta with his wife, Ann, had announced her retirement after the race, but allowed her to remain at trainer John Shirreffs’ barn during the winter rather than ship her straight to Kentucky to begin her life as a broodmare. Shirreffs pulled off her racing plates, and let her grow a winter coat.

But then, Moss had a change of heart. And Zenyatta was back in training, preparing to rewrite history.

Willard says he was happy to postpone retirement to remain with Zenyatta, despite chronic sciatic pain that kept him awake nights. “It really knocked me out,” Willard said during a break in training at Shirreffs’ Hollywood Park barn. “I was sleeping one or two hours a night. I was getting cortisone shots, and that’d help a little bit but it would come back. Vicodin didn’t help none.”

A self-professed conservative who previously renounced anything he perceived to be “New Age-y,” Willard finally found relief through acupuncture and reflexology.

“I figured, that stuff’s been around 10,000 years, let’s give it a try,” he said. “I used to be an extreme conservative; now I see both sides. My dad hated Roosevelt for instituting Social Security. So, you see, it’s drilled into you.”

Willard makes no attempt to hide his affection for the mare he calls “a gift from God.” When they first met, Willard was 63 and Zenyatta was 2. But horses age faster than humans, and the great ones can take years off a man’s life. Maybe, like the characters in “The Curious Case of Benjamin Button,” Willard and Zenyatta have finally “met in the middle.”

“For so long, she was just another good horse,” he said. “They can be bred out of this world, and look great, but if they don’t have it in here (he points to his chest), you’re screwed. She has so much ‘want’ in her. That’s what it takes … the “want.” We’ve never reached the bottom of her … when you hit the front she always wants to give you more.”

Willard says Spencer, his girlfriend of 19 years, had a notion about Zenyatta long before he did. In late 2006, Willard was on the Hollywood training track galloping the unraced 2-year-old when Spencer trotted by on another horse.

“I like that one,” Spencer called out.

Willard shrugged. “Yeah, she’s big, but I don’t know if she’s any good,” he replied.

Willard has perfected an ironic tone in his retelling of the story. “Little did we know, huh?” he said. “I only got her through process of elimination. Three or four riders had tried her before me.”

Willard counted them off. “Frankie didn’t like her; she bounced him off both rails,” he said. “Another girl tried her, got bucked off. Another girl got dumped.”

He laughed. “She’s pretty straightforward now, but ooh, she was a hussy,” he said. “We’ve never raised a hand to her, but we’ve been firm with her. We’ve had to be. She was no kid’s horse.”

Which made her a perfect match for the rider known to all on the backstretch by his nickname: “Grandpa.”

Willard’s affiliation with Zenyatta puts him in exclusive company. Kelso, the five-time Horse of the Year, had Dick Jenkins; Ruffian had Squeaky Truesdale. John Henry reportedly hated everyone except his gallop boy, Lewis Cenicola. Secretariat needed two men to keep him fit — Charlie Davis and Jimmy Gaffney.

And for Zenyatta, it’s Willard — a fact that never ceases to amaze anyone who knew him 15 years ago.

“I was a miserable s.o.b.,” Willard said. “Really, I was an ass. I can admit it. And I still can get that way when something tees me off but I try to get a handle on it.”

At least some of Willard’s irascibility could be blamed on the back pain that he began experiencing in his mid-20s when he was a jockey working the Detroit/Oaklawn/Fair Grounds circuit. Willard began riding horses at a farm in Wadsworth, Ill., in 1960. His childhood had been a fairly privileged one — his father owned two women’s clothing stores in the Chicago suburbs and made a comfortable living — but Willard was a self-conscious kid who had problems with reading and spelling. He found out much later in life that he was dyslexic.

“I was always so embarrassed growing up,” said Willard. “My parents were so frustrated, because my father and brother were consummate readers. It gave me an inferiority complex, is what it did.”

Willard went to Arlington Park in 1963 to gallop horses. “I thought I was going to get real tall, and I wanted to try riding races,” he said. “I thought maybe I could ride through my early 20s.”

From 1965 through the early 1980s, Willard won “500, maybe 600 races.” In the 1970s, he rode horses owned by Southern Pines’ huntsman L.P. Tate at Delaware Park and Timonium, and even spent a week riding jumpers at Tate’s Longleaf Pines farm on Midland Road.

“It was a very mediocre career,” Willard says. “I might have had a better career, but I was a lazy kid. The only reason I became a jockey is because people said I couldn’t.”

In 1984, Willard went to work for trainer Jack Van Berg at old AkSarBen (Nebraska spelled backwards) in Omaha. He began getting on the Preakness winner Gate Dancer, a high-strung colt who needed earmuffs, a shadow roll and blinkers to maintain a straight path in his races. Willard traveled with Gate Dancer to California that year for the inaugural Breeders’ Cup and decided to stay, working for trainers Darrell Vienna and Willard Proctor before finding his way to Richard Mandella’s barn.

For an exercise rider, the Mandella barn was the place to be in the mid-1990s. “We had 10 Grade I winners in the barn at one time,” Willard says. “Gentlemen, Siphon, Sandpit, Dare and Go, Dixie Union … I got on so many good horses.”

The pressure of handling so many high-quality horses began to wear on Willard. “I had trouble delegating responsibility,” he said. “I’d want to do everything myself. I’d tell the guys to do something, and if they wouldn’t do it, I’d throw a temper tantrum.”

In 2001, Willard left Mandella to work for Shirreffs, who would win the 2005 Kentucky Derby with Giacomo, owned by the Mosses. But the pain from his sciatica was hindering his riding, and he began to contemplate retirement.

“Then she came along,” Willard said.

When he first started working Zenyatta, Willard found it hard to gauge the filly’s ability. Maybe that’s because she rarely allowed him access to it.

“She could do handstands,” Willard said. “When she bucked … you see that up there?” (He pointed to a rain gutter on the barn roof.) “Her back feet would be that high. She’d get perpendicular. John would say, ‘I don’t ever want to see that again.’”

Willard had learned to “cowboy” a horse while working with trainer Don Peebles in Illinois. He saw that the best way to ride a bucking horse was to “just kind of go with it, sit into it.”

“I was working with some really good horses at Don’s place,” Willard said. “And some really good horsemen. I learned what you should know about horses … what made them tick.”

Once Willard was able to harness Zenyatta’s energy, he and Shirreffs were able to see what they had. They soon learned that, because of her size, Zenyatta was a horse that would take several strides to get in gear.

“She’s sort of slow motion,” Willard said. “When we took her to the gate the first few times, and she’d break out slow, we knew she wasn’t going to have any speed. We knew we were going to have to wait for her to get underneath herself to get going.”

Willard rode Zenyatta for a year before the filly was ready to race. By then, the veteran rider suspected he might be on something special.

“I’d worked her behind horses a ton of times, just let her eat dirt, let her eat dirt, and she’d just run into it,” Willard said. “The last work before her first race, I waited, and she waited, and I asked her to go. BOOM! She was gone.”

Willard shook his head. “I don’t bet on many horses,” he said. “But I bet $200 on her the first time she ran.”

Shirreffs has never hesitated to credit Willard and Zenyatta’s groom, Mario Espinoza, with keeping Zenyatta in peak mental and physical condition. It hasn’t been easy — Zenyatta can still be a handful, and now she’s a 17-hand, 1,250 lb. handful.

“When we took her to Oaklawn (for the Apple Blossom) the first time, John wasn’t there so I was schooling her with Mario,” Willard recalled. “She came out of the paddock and saw the track, and she just did this number … she kind of blew up. Mario only had to walk her about a sixteenth of a mile, but he had to keep stopping because she was lifting him off the ground. And he weighs 220 lbs. I couldn’t help, because I’m 118 lbs. and she’d throw me around like a rag.”

Zenyatta’s Rockette-inspired march in the paddock and post parade attracts almost as much attention as her races. Willard says the march evolved as Zenyatta began to intuit when she was racing.

“She always pranced,” he said. “Then all of a sudden she started to do the walk, all on her own. It’s a dressage move. She’s venting … it’s anticipation.”

As Zenyatta’s career winds down, so too does Willard’s. The mare will run in the Oct. 2 Lady’s Secret Stakes at Hollywood Park before defending her title in the $6 million Breeders’ Cup Classic at Churchill Downs on Nov. 6.

Willard is convinced that Zenyatta would not have become the superstar she is today under the management of anyone other than Shirreffs, whom he teasingly refers to as “the Nebraska farmer.”

“I can’t think of one other trainer that would have given her the time John’s given her,” Willard said. “Yeah, we got a good horse to start with, but the way John has managed her and the way Mario’s taken care of her … impeccably, is the only word I can come up with.”

If Willard has one regret as Zenyatta heads into retirement, it’s that a showdown with 2009 Horse of the Year Rachel Alexandra never transpired. Rachel has lost three races this year, and her connections have suggested they may not run her in any Breeders’ Cup races.

“Rachel don’t want no part of Zenyatta,” Willard said. “Never has. I said last year, I don’t have a lot of money, but I’ll put up $10,000 or $20,000 of my own money that no matter where Rachel finishes, we’ll finish ahead of her. That’s how much confidence I have in Zenyatta.”

The mood at the Shirreffs barn is upbeat but melancholy, as employees prepare for the big mare’s departure. And Willard isn’t the only one who plans to retire with Zenyatta: Her 20-year-old pony, Hootie, is also calling it a career. Hootie’s owner, Freddie Wilson, has hinted at cashing in on Zenyatta’s celebrity and listing the sorrel quarter horse gelding on eBay.

“Two more races,” Willard said. “That’s all she’s got to do. We say that over and over.”

As bittersweet as it will be, Willard is excited to see what Zenyatta might produce as a broodmare.

“One thing’s for sure,” Willard said with a grin. “Whatever stallion they choose, he’s gonna need stilts.”