Soon appointed Newsday’s horse-racing writer, he met Secretariat, then 2 years old, in the summer of 1972. Jimmy Gaffney, the exercise rider for Riva Ridge, who had just won that year’s Kentucky Derby and Belmont Stakes, ushered Mr. Nack to Secretariat’s stall.
“Don’t quote me,” Mr. Nack recalled him saying, “but this horse will make them all forget Riva Ridge.”
The encounter began a journalistic love affair between Mr. Nack and Secretariat, a chestnut-colored colt with three white feet and a white blaze down his face. Starting in March 1973, Mr. Nack spent 40 consecutive days with Secretariat, joining him at 7 a.m., getting to know his team, taking copious notes and once seeing the horse playfully grab his notebook in his teeth and deposit it on a bed of hay.
Mr. Nack covered Secretariat’s victories at the Kentucky Derby and the Preakness Stakes. Then, on the morning of the Belmont Stakes, June 10, 1973, he arrived at Belmont Park at 2 a.m., dozed off in Barn 5, where Secretariat’s stall was, rose to a rooster’s crowing and heard the stable foreman shout, at just past 6 a.m., “Get the big horse ready!”
About 12 hours later, Secretariat won in record time.
“It ended with a single stentorian burst of applause, with screams so sudden they seemed startled out of people,” Mr. Nack wrote in Newsday, “and they began when Ron Turcotte pushed Secretariat to ever-widening leads of 28, 29, 30 and finally 31 lengths in the Belmont Stakes.”
Mr. Nack went on to write the horse’s biography, “Secretariat: The Making of a Champion” (1975), which was the basis of the 2010 film “Secretariat,” directed by Randall Wallace. By then he had become so familiar with Secretariat — and so identified as his Boswell — that he knew tales about the horse as well as he “knew the stories of my children,” he wrote in Sports Illustrated in 1990.
“Told them at dinner parties,” he added, “swapped them with horseplayers as if they were trading cards, argued over them with old men and blind fools who had seen the show but missed the message.”
He wrote about many other horses as well, including Ruffian, who broke down during a match race against Foolish Pleasure in 1975.
In an interview for the website Still No Cheering in the Press Box in 2012, Mr. Nack recalled running more than 600 yards to where Ruffian had fallen and finding the doctor, his hands bloodied, fitting the horse’s leg with an inflatable cast. She had fractured bones in her right foreleg. Mr. Nack wrote that they had “exploded out like little hand grenades.” Ruffian was euthanized.
“It was the worst thing I’ve ever covered in my life,” he said.
The incident was retold in his book “Ruffian: A Racetrack Romance” (2007). In The New York Times Book Review, an editor and horse-racing writer, Eric Banks, observed, “Some might scoff at describing the demise of a horse (and all she symbolized) as a tragedy, but Nack’s requiem — for the animal, for his feelings — summons nothing less.”
William Louis Nack was born on Feb. 4, 1941, in Chicago. His mother, the former Elizabeth Feeney, was a ballerina, and his father, Gordon, was an electrical engineer.
After the family had moved to Skokie, Ill., Bill and his sister, Dorothy, began riding horses and cleaning out stalls at a riding stable in nearby Morton Grove in the early 1950s. In the summer of 1959, Bill was a groom at Arlington Park and other tracks in the Chicago area. He attended the University of Illinois, where he was sports editor of The Daily Illini, the campus newspaper, and then its editor, succeeding the future film critic Roger Ebert.
Mr. Ebert was one of many colleagues who recalled Mr. Nack’s penchant for reciting from “Lolita,” “The Great Gatsby” (in English and Spanish) and some works of H. L. Mencken’s.
“He approached literature like a gourmet,” Mr. Ebert wrote on his website in 2010. “He relished it, savored it, inhaled it and, after memorizing it, rolled it on his tongue and spoke it aloud.”
While serving in the Army during the Vietnam War, Mr. Nack wrote speeches and news releases in Saigon for Gen. William C. Westmoreland, the commander of United States forces from 1964 to 1968. He left Vietnam six weeks after the Tet offensive began in 1968, his plane taking mortar fire before it took off.
On returning, he joined Newsday and stayed for a decade before Sports Illustrated hired him.
“His love of the subject came through in his writing,” Chris Hunt, the former articles editor for Sports Illustrated, said in a telephone interview. “He knew how to set up a story and carry it to the end.”
Mr. Nack left Sports Illustrated in 2001 and contributed to ESPN and GQ magazine. He won seven Eclipse Awards for excellence in thoroughbred writing and, last year, the PEN/ESPN Lifetime Achievement Award for Literary Sportswriting.
In addition to his wife, the former Carolyne Starek, he is survived by his daughters, Emily, Rachel and Amy Nack; a son, William; six grandchildren; and his sister, Dorothy Consigny. His marriage to Mary Scott ended in divorce.
Mr. Nack returned to Secretariat, his favorite subject, to tell the story of the horse’s death. On a visit in October 1989, to Paris, Ky., where Secretariat lived on Claiborne Farm, he learned that the horse, afflicted with the hoof disease laminitis, had to be euthanized.
News of the horse’s death reached him when he was back in his hotel room.
“That last time I remember really crying was on St. Valentine’s Day of 1982, when my wife called to tell me that my father had died,” he wrote in a long article in Sports Illustrated. “Now, here I was, in a different hotel room in a different town, suddenly feeling like a very old and tired man of 48, leaning with my back against a wall and sobbing for a long time with my face in my hands.”