Ball person Anna Kuligowski hands a towel to Steve Johnson during his quarterfinal victory over John Isner on Friday at the Citi Open. “We try to keep it as fun and light as possible,” said the tournament’s ball person chair, Joey Ramsey. (Jonathan Newton/The Washington Post)

In the blazing July heat, it’s not just tennis players taking their positions on court at the Citi Open in Rock Creek Park. Donned in black T-shirts, black shorts and white hats, 130 ball persons spend roughly 100 hours volunteering their time over the course of the 10-day tournament, and for half of that — 50 hours — they’re on the court.

On center court Friday afternoon, that meant scurrying after balls on the hardtop surface where the official thermometer registered 128 degrees shortly before a quarterfinal match between Americans John Isner and Steve Johnson got underway.

Garth Herbert, 21, worked the near two-hour contest, which was won by Johnson in two sets. Afterward, Herbert needed a new black T-shirt — and a well-earned cooldown in the air conditioned trailer set aside for ball persons during the event.

“It was hot out there,” Herbert said. “Luckily, there’s a huge emphasis on hydration here. We all drank plenty of water before we went out there, so we were okay for the most part. . . . With these all black clothes, there’s sweat. We’re all sweating out there. We bond over the intense heat.”

Which for ball persons at the Citi Open is part of the fun.

Joey Ramsey, the ball person chairperson, started out as a ball person when he was 12 in 1999. Even though the age limit was increased to 14 the following year, Ramsey was grandfathered in. And he has loved it ever since. He spent nine years as a ball person and eight as a chairperson, coordinating logistics.

“It’s cool seeing this progression,” Ramsey said of the younger ball persons. “They grow up, they become different people. It’s so cool to see. I really love teaching.”

About the name: Don’t call them ballboys. Don’t call them ball kids. They aren’t all boys, and they aren’t all kids. Even though the minimum age is 14, some have been on the court for over 25 years.

Stuart Berlin of Rockville, some 25 years ago, said to his wife that being a ball person looked like fun. After talking with a friend who had volunteered as an usher, he was all in.

“She said you just volunteer and you show up and it’s as simple as that,” Berlin said. “And it was as simple as that.”

The process of being selected is slightly different now, as there are tryouts for the position and throughout the week, ball persons can get cut or dismissed as the number of matches played each day dwindles. On Friday morning, 63 ball persons remained.

“I feel like I’ve earned my spot,” Berlin said. “It’s not just that they keep one warm for me.”

Throughout the years, Berlin has missed a few years because of various things that have come up, but regardless, he’s a tournament mainstay. “I was with some kids yesterday, and they were like, ‘He’s been doing this longer than we’ve been alive,’ ” Berlin said.

Berlin’s daughter, Debbie, got involved as soon as she was old enough. Debbie has been on the court for eight years now, though this season was a little shorter as she had spinal surgery Monday. Berlin said his daughter is doing all right.

One of the younger ball persons, Sydney Wood, 15, has watched the tournament since she was little. This is her second year. She was hoping to make it past Friday cuts, as that’s where she made it to last year, and onto this weekend’s men’s and women’s semifinals and finals. Through her two years, she has made a few memories, a lot of them resulting in tennis ball-size bruises.

“I’ve gotten hit twice by Isner’s serves,” Wood said of the 6-foot-10 player whose serves can exceed 140 mph. “I actually have a bruise right now.”

She got hit by a Jack Sock serve Thursday too. Sock apologized and asked whether she was okay. Wood said at least five ball persons got hit by serves Thursday alone.

Ball persons come onto the court in teams of six, and they have different positions on the court. Two cover the net, while four take care of the back of the court.

“The nets are often doing more of the running back and forth, whereas the backs have a lot to keep track of,” Ramsey said. “[The backs] will do a good amount of running in their little area, like getting the player their towel. . . . It is plenty of exercise.”

Artavius Lincoln, 24, primarily works on the net. And while coming in each day after work, he has logged nearly 20 miles on his Fitbit for the week.

“I only wear it here,” Lincoln said.

But it’s not solely physical.

“It is as much mental as physical,” Ramsey said. “We hold them to a really high standard, especially here later in the week. We’re expecting them to keep track of all six balls in their head. . . . There’s all this thinking and tracking that goes on and anticipation and knowing what to do next [that] makes everything so smooth.”

And it’s not all work either.

The ball persons — and the rest of the volunteers at the tournament — are well taken care of. The ball persons work one-hour-on, one-hour-off shifts. And for changeovers, they can get subbed out for a water break.

During that hour off, they can hang out in their air conditioned trailer, the volunteer tent with food and water, watch tennis matches, walk around or a little of all of the above.

On Thursday evening in their trailer, a handful of ball persons were hanging out, listening to assorted songs from Blink 182’s new album. On Friday morning, several sat around a table in the volunteer tent playing the Settlers of Catan board game.

“We try to foster this atmosphere where it’s like summer camp,” Ramsey said. “We’re going to go out there and do a really great job, and they really care about this minutiae of this ridiculous thing of picking up balls in the most efficient way, but at the same time, we try to keep it as fun and light as possible.”

And that’s why everyone involved keeps coming back.

“I enjoy the people and the kids,” Berlin said. “I’m energized by them. The more time I hang around them, the more comfortable they are to have someone outside their age-group sweet spot. They’re a good group. It’s fun.”