FOR DECADE upon decade, if you wanted to become a devoted sports cartoonist, as I did, you had to reckon with the talent of one man. When making the stations of the crosshatch, you had to understand the visual vocabulary of Jack Davis.

Davis, who died Wednesday at age 91, received formal art training, yet he also had the rare ability — not unlike a great athlete — to innovate and seemingly even improvise between the lines. It was that balanced marriage of formality and quick-penned finesse that gave his work a perfect verve. The confident stroke that feels both instant and forever true.

To study at the feet of Davis, for instance, you literally have to study his rendered feet — those wonderful, clown-shoe creations so often in big, bent cleats. Even the most graceful Davis jock had the exaggerated leg motion as if running in diving fins. And in that pen-and-ink hyperbole, there was signature music.

Davis was proudly a southerner, and even when he drew his first University of Georgia students sports cartoons, there was a structural depth, as if he were sculpting in space.

“As a kid, the first cartoonist I had a crush on was Jack Davis,” Art Spiegelman, the Pulitzer-winning cartoonist (“Maus,” RAW), tells The Post’s Comic Riffs. “I would study a Davis drawing, and everything is tucked in behind something else. It has this strangely dimensional quality.”

In the early 1950s, Davis became a founding contributor to MAD magazine, where he worked for many decades — and where many future talents first discovered him as kids. He could do it all, from celebrity caricatures and Hollywood parodies to spoofs of the media and the burgeoning suburban lifestyle.

“It knocked me out, seeing those MAD paperbacks,” Spiegelman says. “His lines didn’t look like anybody else’s.” In an era when such comic-strip creators as Alex Raymond and Milton Caniff had spawned imitators, Davis was utterly distinctive.

Davis left MAD for several years, when the humor began to get more “out there” than Davis, a “true southern gentleman,” was comfortable with, says MAD art director Sam Viviano. But Davis — whose high-profile clients included Time and TV Guide — returned in the mid-’60s and became a mainstay.

“Jack had a lot of imitators over the past 65 years,” Viviano tells The Post’s Comic Riffs, “but what they couldn’t imitate was his spirit, his heart — that true mark of any great artist.

“Jack drew that way because Jack saw that way,” Viviano continues. “And his work just flowed out of him, as if he were possessed. One of the things that really appealed to me was that it had a slapdash look to it, even though it was extremely sophisticated.”

Those gifts served Davis well, whether he was creating Hollywood movie posters (including “Bananas” and “The Bad News Bears”), covering national newsmagazines or rendering sports cartoons.

“It was that combination of the sophisticated understanding of art technique and yet an incredible, innate sense of humor and joy,” Viviano says. “And how much he loved sports, particularly football, and his understanding of it and his love of it showed in his work. It’s no puzzle why he got so many sports articles assigned to him.”

“If there is a pantheon of cartooning, Jack Davis would be at the head of the table,” MAD artist Tom Richmond says. “Jack was one of those artists who did everything better than anyone else and yet remained one of the humblest people in the business.

“It would be difficult to come up with a venue of cartooning where Jack Davis didn’t do some kind of significant work … ,” Richmond continues. “Jack could do it all and make it look effortless. He had one of the most distinctive styles in cartooning, and had the rare gift to make a viewer laugh with nothing but the visual magic of his art … no gags required. There will never be another like him.”

Davis, who was born in Atlanta on Dec. 2, 1924, received the National Cartoonists Society’s lifetime achievement award in 1996; nearly a decade later, he was inducted into the Society of Illustrators Hall of Fame.

He remained warm and humble and profoundly talented.

“He was,” Viviano says, “the role model for us all.”