Yeaaaaahhh … you can’t do that.

In the end, it doesn’t matter if Novak Djokovic meant to do it or not—and he says he wasn’t trying to hit a line judge with a tennis ball at the U.S. Open on Sunday—you just can’t do that.

Djokovic struck a ball in frustration, the ball hit a judge, she frighteningly tumbled to the ground, and even if he didn’t see her there, or says it was an accident, a terrible mistake, it’s still considered a punishable no-no—enough in the minds of tournament officials to send Djokovic, the Open’s No. 1 seed, shockingly home.

“We all agreed he didn’t do it on purpose, but the facts are still that he hit the line umpire,” the Open’s tournament referee, Soeren Friemel, said at a post-match news conference. “And the line umpire was clearly hurt.”

In case you’re mounting a furious defense of Djokovic in the Court of Tennis Court Second Opinions, be advised: Your client Nole has pleaded nolo.

In a contrite Instagram statement issued after his default, the World No. 1 offered no excuses.

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“This whole situation has left me really sad and empty,” Djokovic’s statement read. “I checked on the lines person and the tournament told me that thank God she is feeling ok. I‘m extremely sorry to have caused her such stress. So unintended. So wrong.”

“I need to go back within and work on my disappointment and turn this all into a lesson for my growth and evolution as a player and human being. I apologize to the [U.S. Open] tournament and everyone associated for my behavior.”

As sports apologies go—as any apology goes—that’s not bad. What happened Sunday was a self-inflicted professional humiliation, and credit to Djokovic for not doing the 21st-century usual: blaming everyone but himself.

What a weird, pandemic-edition U.S. Open. No fans, no towel service, no Rafa, no Roger, some scattered drama over Covid-19 testing, and now, Novak hitting the bricks with an epic unforced error. There is a silver lining—the men’s singles winner will be a first-time major winner—but the first Covid-era tennis major has faced substantial headwinds. (If you work for the USTA, and you’re up to four Honey Deuces a night, I understand.)

I know there are folks pointing out an apparent irony: that the pandemic U.S. Open is using the “Hawk-Eye Live” robotic line-calling service to officiate matches on most of its courts, in order to cut down on the number of people required to put the tournament on, and had Djokovic been playing his 4th round match against Pablo Carreño Busta on any court besides Arthur Ashe or next-door Louis Armstrong Stadium—the two courts that are still using old-fashioned humans—he wouldn’t be in this situation.

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Which I guess is true. You know what’s also true? Novak Djokovic doesn’t play tennis matches at the U.S. Open out in the sticks. He’s not playing Hawk-Eye Live on Court 13.

Djokovic is the best men’s tennis player in the world, and has been for a while. He’s won five of the last seven major tournaments—Wimbledon 2018, U.S. Open 2018, Australia Open 2019, Wimbledon 2019 and Australia 2020—and is up to 17 majors, two behind Nadal (19) and three behind Federer (20). He’s got a lifetime winning record against both of those all-timers.

He flies in the highest possible airspace. Only a fool would dispute the depth of Djokovic’s talent, his all-around skill, his outrageous athleticism and court coverage.

And yet Djokovic is consistently put in a corner next to Nadal and especially Federer. His matches against the latter have a distinct WWE vibe, with Djokovic installed as the heel. While some of this tension is overbaked—everyone plays the heel when pitted against the wildly-beloved Federer—it is awkward to watch the historically great Djokovic reduced to such easy villainy.

Which is why Sunday was a disaster, and he knows it. With Roger icing his knee and Rafa prepping for the French, this U.S. Open men’s tournament had the potential to be The Novak Show, a stage largely to himself. He’d been playing very well (he’d won the Western & Southern tune-up, and had dropped one set thus far in the Open) and appeared to be enjoying his rabble-rouser status as a leader of an upstart players organization called the Professional Tennis Players Association. The folly of Djokovic’s summer tennis exhibition in Serbia and Croatia, where he and several other players tested positive for Covid-19, appeared to be fading. He was a heavy favorite in New York, undefeated (26-0) on the year.

All of which makes this such an unnecessary fiasco. Djokovic’s default was the kind of viral moment everyone heard about, even if they don’t pay the slightest bit of attention to tennis, or to sports at all. Suddenly, he was back to Novak the Heel.

Here’s the truth about heels: Heels only work when they want to be heels. Djokovic wants to be loved. He’s a temperamental player with a self-destructive side, but he also has the potential to finish his career with more majors than any men’s player ever. I’ve seen few athletes with a greater capacity to dig deeply in pressurized moments, yet at this point in his arc, he needs to locate more grace in his game. I suspect he realizes this. I keep going back to a sentence in Djokovic’s apology: I need to go back within and work on my disappointment and turn this all into a lesson for my growth and evolution as a player and human being. In the wake of a mortifying moment, it’s valuable self-awareness. May it turn into wisdom.

Write to Jason Gay at Jason.Gay@wsj.com

Corrections & Amplifications
Rafael Nadal has won 19 major singles titles. An earlier version of this article incorrectly said he had won 18. (Corrected on Sept. 7)