Linesman Ricardo Durham concentrates on the action as Caroline Garcia returns a shot at the Citi Open. (Jonathan Newton/The Washington Post)

The third-set score was knotted at four games apiece when Anna Tatishvili blasted a deep forehand past Caroline Garcia during their first-round match at Washington’s Citi Open.

“Out!” a line umpire shouted.

“Correction!” the chair umpire barked a millisecond later. “The ball was good. Replay the point.”

The veteran linesman, Ricardo Durham, displayed no embarrassment as the point was contested anew, just as he made no outward show of victory less than three hours later, when the Hawk-Eye replay system on Stadium Court, the tournament’s biggest stage, affirmed his correct call on a 128-mph serve by American Sam Querrey.

It was all in a day-and-night’s work for one of the least understood jobs in tennis.

At first blush, being a line umpire for a tennis tournament looks like a job anyone could do — demanding the ability to stare at a line while sitting or standing, good vision and a vocabulary of four tennis terms: “out,” “fault,” “foot fault” and “correction,” as well as the vocal cords to shout them with authority.

But it is far more difficult.

“Everybody thinks it’s just about vision,” says Joan Vormbaum, the chief umpire for the Citi Open, “but everybody has 20-20 vision. It takes decisiveness. Courage. A special gift of timing. Anticipation. Quick reflexes. This is not for the faint of heart.”

It takes 80 line umpires to stage the Citi Open.

The job of scheduling them falls to Vormbaum, herself a former linesperson and chair umpire. With more than three decades of experience, there is no facet of the job Vormbaum doesn’t know intimately. And there is no aspect of line umpires’ needs that she doesn’t tend to — including scheduling airport pickups and drop-offs, arranging transport between the hotel and tournament grounds, ordering meals and taking in bags of dirty clothes for pickup by an overnight laundry service.

“Happy people make good calls,” she says.

Most importantly, she assigns linespeople to matches, organizing them in multiple crews of seven that report to crew chiefs, who assign their crews to specific lines. Each crew rotates among the courts, working one-hour shifts interspersed with 40-minute breaks to rest their eyes.

Each crew reflects a range of experience, with the more seasoned officials typically assigned to service lines and the sideline that is farthest from the chair umpire. After each tournament, the chair umpire grades each linesperson on a scale of 1-7, 7 being the highest score, with the grades influencing their assignments in subsequent matches.

To get a top grade, a line umpire must be tested by difficult calls during a shift and handle them well. Big serves and groundstrokes that skim the lines establish a linesperson’s bona fides. Through Tuesday, the line calls of Citi Open line umpires were upheld upon Hawk-Eye review 85 percent of the time for women’s matches and 50 percent for men’s matches.

On the Citi Open’s busiest day, with all five courts in use, the linespeople report to the officials’ tent at 1 p.m., one hour before play begins, to check their assignments, grab lunch and prepare for a work day that may stretch until midnight.

It looks like a middle-aged scout troop, with men and women alike in full uniform: khaki shorts, navy polo with a Corona beer logo and navy cap. Each has been certified in the rules of tennis.

They are professionals, paid roughly $100 a day for their expertise. Some, like Durham, hold full-time jobs in addition. Others eke out a living traveling the world calling lines. Their hotels and meals are paid for, and each gets a modest travel stipend.

Just a few decades ago, it was a strictly volunteer job performed by tennis enthusiasts of varied skills. It was no wonder that amid a particularly poorly called match in 1979 that John McEnroe flung himself onto a court face first, bashed his racket and screamed, “I can’t believe I do this for a living!” It was hardly the most notorious of McEnroe’s eruptions.

Vormbaum recalls being compensated in the early 1980s with a $20 bill, a Coke and a hot dog.

But as the prize money in pro tennis skyrocketed, the sport recognized the need for professionally trained officials.

Many are former recreational players. Some never played. But all share a lifetime love of the game and a bedrock belief in fairness.

As the linespeople make their final preparations before marching out to their first rotation of the day, slathering on sunscreen and, in some cases, putting in contact lenses, Vormbaum issues her final directive: “Let’s have a great day!”

Durham’s first assignment, a women’s match on Grandstand Court 2, is cut short after one of the players injures a hamstring and can’t continue.

He heads back to the air-conditioned officials’ tent, where several of his peers are playing cards.

“It gives you a chance to close your eyes or do something different,” Durham says of the scheduled breaks. “You don’t want to think about tennis on your break after thinking about it on court. Your head might explode.”

On court Durham makes his calls with authority, extending his right arm as he shouts something that sounds more like “H-out!” than “Out!” By adding a slight “H” sound, the word explodes with more force.

“Conceptually what you’re trying to do is show people, ‘I’m certain of my call,’ ” Durham says. “You use your voice to indicate that confidence you have. The closer the call is, the more authority you try to use in your voice to say, ‘I know it was close, but I really did see it.’”

Afterward, Durham declines to discuss the call that was overruled. That, too, is part of the job: keeping any opinions about specific calls, players or match situations to yourself.

He is no more talkative about the call that Hawk-Eye affirmed.

After a decade calling lines, Durham insists he has no “history,” whether good or bad, with any player. The job of calling lines, he explains, has little to do with players.

“The only thing that’s important is whether the ball is hitting the line,” Durham says. “Your opponent, in this context, is the ball and line. So you have to focus on your opponent. If you get caught focusing on the player, then you end up someplace else.”