The Hardest Ticket at the U.S. Open? Ball Person.

Thirty minutes before the gates to Arthur Ashe Stadium opened at 4 p.m. on June 22, a cluster of people began gently rolling tennis balls across a parking lot.

One after the other, they lowered one knee close to the ground, extended their opposite arm, and released a tennis ball to someone 10 feet away.

Closer to a locked chain-link fence, a gaggle of people started doing calisthenics as others nervously shifted their weight back and forth, tightly clutching their paper applications.

The group of 500 people — already whittled down from some 1,200 online applicants — would be vying for 120 spots as ball people for the U.S. Open, during tryouts over a full week that were forced indoors because of rain. Those selected joined the 200 or so ball people who are returning to service the courts in Queens.

“I don’t think people understand, it’s a highly sought-after job,” said Tiahnne Noble, the director of the U.S. Open Ball Crew.

Ranging in age from 14 to their mid-60s, the hopefuls came from all corners of the country. Applicants flew in from California, drove from Indiana, took the subway from the Bronx and rode the train from Connecticut. Some were tennis fans, some played themselves and others had their curiosity piqued from seeing ball people on TV. Could they do that? (Spoiler: Mostly not.)

The adults were generally far more anxious than their younger counterparts. The experience was described as a “dream” by many over the age of 30.

Masami Morimoto, 59, said she had been determined to try out before she turned 60. “I love tennis,” the Manhattanite said, a bounce in her step. “I couldn’t sleep, I was so excited.”

Groups were led through a series of drills during 30-minute auditions, in which they were asked to quickly and quietly roll, retrieve and toss balls. Participants were locked in, acting as though, at any moment, Novak Djokovic would look one of them dead in the eye and motion for a ball.

Supervising staff members were highly attuned to the jitters. When an applicant forgot instructions, throwing a ball instead of rolling it, they were quick to comfort the anguished applicant. “Don’t worry about it!” they would say gently, sending a tennis ball back in their direction.

Body language suggested the message went unheeded.

Noble and her staff of veteran ball people said they could spot a potential ball person almost immediately. Ball people, she said, must have speed, agility, quick reflexes and the ability to blend into the background of the tournament.

Six ball people work each match, communicating clearly and silently so as not to distract the pros or spectators. They need to respond to the preferences of different players — some only want a ball tossed to them with their left hand, for example — and act as invisible guardians of the game.

As evaluators looked on at the tryouts in June, there was plenty of subtle nodding and note taking on clipboards.

The auditions were not for the faint of heart. “It’s the U.S. Open,” Aaron Mendelson, 57, said with a deadpan acknowledgment of the stakes. He rolled a suitcase alongside him, having flown in from San Francisco for the occasion. He said he planned to head straight to the airport afterward.

Mendelson knew what to expect. He had been a ball person at the 1992 U.S. Open, working the match between Jim Courier and Andre Agassi. He pulled up a YouTube clip as proof. “Look for the redheaded kid,” he said.

Applicants would not know if they had been selected for another week, but some were already cautiously sketching out plans for where they would stay. While the U.S. Open is the only Grand Slam to pay its ball people — $16 an hour for most recruits — it does not provide housing. “Which borough would you recommend?” Avani Kondragunta asked this reporter.

Her 21-year-old daughter, Alekhya, had previously been a ball person at the Western & Southern Open near their home in Cincinnati. So the two decided to make the 10-hour drive for tryouts.

As the high-stakes auditions drew to a close, prospective ball people shuffled off the court sweaty and shrugging their shoulders. They would receive an email with their acceptance — or a rejection — soon enough.

“It wasn’t too hard,” said Debra Gil, 14, of the Bronx as she walked off the court. She was one of the youngest applicants with experience under her belt. Her brother had been a ball person the previous year, and she had worked the Bronx Open.

After Mendelson finished his tryout, he stumbled upon another group of Californians who had traveled in for the opportunity. The father-daughter duo Kuangkai and Emily Tai of San Diego had both tried out.

When asked whether, if selected, they would return for the duration of the U.S. Open, Emily Tai, 19, responded with a cautious, “We’ll see!”

Her father’s eyes bugged. “Oh, we’re coming.”

“If you pay,” Emily responded.

Of those interviewed, only Emily Tai got the golden ticket — erm, email. She was surprised to have made the cut over her father. “He’s in way better shape than me,” she said.

Kuangkai Tai planned to stick to his word. Though he will not be servicing courts, he plans to come watch his daughter.