The toughest ticket at the US Open? Ball person.

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

One by one, they lowered one knee near the ground, extended their opposite arm, and launched a tennis ball to someone 10 feet away.

Near a closed mesh fence, a group of people began to exercise while others nervously shifted their weights back and forth, holding their paper applications tightly.

The group of 500 people – already whittled down from about 1,200 online applicants – will compete for 120 places as pitchers at the US Open, during a week-long tryout that forced them indoors due to rain. Those selected join the 200 or so ballplayers who have returned to court duty in Queens.

“I don’t think people understand, it’s a job that’s in high demand,” said Tian Noble, director of the US Open Soccer team.

Ranging in age from 14 to mid-70s, hopefuls came from all over the country. Applicants traveled from California, drove from Indiana, took the subway from the Bronx and took the train from Connecticut. Some were tennis fans, some played themselves, and others were intrigued by seeing ballplayers on television. Can they do that? (Spoiler: Mostly not.)

Adults were generally much more anxious than their younger counterparts. Many described this experience as a “dream” for those over 30 years old.

Masami Morimoto, 59, said she was determined to try the game before she turned 60. “I couldn’t sleep, I was so excited.”

The groups were led through a series of exercises during 30-minute tests, in which they were asked to roll, retrieve and throw balls quickly and quietly. The participants were closed in on themselves, acting as if, at any moment, Novak Djokovic would look dead in the eye and move toward the ball.

The supervising staff was very attuned to the tensions. When the applicant forgot the instructions and threw the ball instead of rolling it, they rushed to calm the upset applicant. “Don’t worry about it!” They said sweetly, sending the tennis ball in their direction.

Body language indicated that the message was not being heeded.

Noble and her staff of veteran ballplayers said they can spot a potential ballplayer almost immediately. She said that football players must have speed, agility, quick reactions and the ability to blend into the tournament background.

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

As the evaluators went through the tests in June, there was a lot of subtle gesturing and clipboard note-taking.

The tests were not for the faint of heart. “It’s the US Open,” said Aaron Mendelsohn, 57, with a deadpan acknowledgment of the risks. He rolled a suitcase beside him, having flown in from San Francisco for the occasion. He said he planned to go straight to the airport after that.

Mendelsohn knew what to expect. He was a bowler at the 1992 US Open, refereeing the match between Jim Courier and Andre Agassi. He pulled a YouTube clip As proof. “Look for the red-haired kid,” he said.

Applicants wouldn’t know if they’d been selected for another week, but some were already cautiously making plans about where they’d stay. While the US Open is the only Grand Slam tournament that pays fans — $16 an hour for most participants — it does not provide housing. “What area do you recommend?” Avani Kundragunta asked this reporter.

Her daughter, Alekhia, 21, was previously a ballplayer at the Western & Southern Open near her home in Cincinnati. So the two decided to make the 10-hour trip to take the tests.

As the high-stakes tryouts came to a close, the would-be prospects walked out of the field sweating and shrugging their shoulders. They will receive an email with their acceptance – or rejection – shortly.

“It wasn’t too hard,” said Debra Gill, 14, of the Bronx, as she walked off the field. She was one of the youngest applicants with experience under her belt. Her brother was a ball player the year before, and she was working the Bronx Open.

After Mendelsohn completed his experiment, he found another group of Californians who traveled for this opportunity. Father-daughter duo Kwangkai and Emily Tai of San Diego tried out.

When asked if they would return for the duration of the US Open, if selected, Emily Tye, 19, responded cautiously: “We’ll see!”

Her father’s eyes filled with tears. “Oh, we’re coming.”

“If you pay,” Emily replied.

Of those interviewed, only Emily Tai received the golden ticket, via email. I was surprised that she cut her father off. “He’s in better shape than I am,” she said.

Kuangkai Tai planned to stick to his word. Although he will not be serving the courts, he plans to come watch his daughter.