PARIS — Tennis, unique in many ways, has differentiated itself from other professional sports in another aspect: With kids on its courts during matches.

Those are the ball kids, of course, the individuals assigned to retrieve loose balls for the players, help them with their towels at the back of the court or hold an umbrella for them at a changeover or between sets on a sunny day.

They are silent personal assistants, in a way, tasked to keep the game move quickly – and seamlessly – along.

Sunday, one played a key role in a first-round match at the French Open, when Nick Kyrgios, the dynamic and outspoken Australian who has sparked plenty of opinions in tennis, yelled at one when the ball boy appeared not to have heard Kyrgios.

“Towel!” Kyrgios shouted to the ball boy toward the end of the first set. He was asking for his towel at the backcourt after not being heard when he asked for it the first time.

Chair umpire Carlos Ramos, watching it unfold, issued Kyrgios an unsportsmanlike code of conduct warning, a ruling that Kyrgios strongly (and loudly) disagreed with.

"Now I've really seen it all," a frustrated Kyrgios said to Ramos after being handed the code violation. "What rules am I breaking?"

It’s a delicate dance, that of player and ball kid: Players are at their most intense on court and along with them are their quiet assistants, fetching balls and towels and bottles of water.

Is it hard to balance? Kyrgios was asked.

“Yes and no,” said Kyrgios, who beat Marco Cecchinato in straight sets. “I think the kids do a great job, especially (at) the Grand Slams. It's usually up to (par). They did a great job today. I'm not even sure if all of them speak English. I might have to be a bit more patient.”

In 2015, during the Miami Open final, a ball kid was caught in the crossfire of a Novak Djokovic scream of frustration directed at Djokovic’s player box. The ball boy, outwardly alarmed, was handing Djokovic his towel, which the Serbian swiped as he yelled.

Later that day, having won the title but also having been met with a stream of social media messages over the apparent mistreatment of the ball boy, Djokovic issued a minute-long apology on YouTube.

“I want to reflect on a bad moment that happened in the final,” the world No. 1 said into the camera. “When I lost the second set, I yelled to my camp in my box in frustration. (When) I saw the replay and unfortunately a ball boy was in the middle of it, I feel sorry and I regret that he was there. There absolutely was no intention to hurt him or scare him in any way. I hope that he forgives me.”

It was clear Kyrgios was yelling at the ball kid, whether or not he should have been given a code violation is up for debate.

“For me … I didn't get mad at all, I just said it a little bit loud,” Kyrgios said.

Ball kids can bring out the lighter – and softer – side of the athletes, as well.

In January at the Australian Open, a ball girl was hit in the face with a ball and Jo-Wilfried Tsonga, noticing she was struggling, put his arm around her as he called her to be taken off the court.

There have been plenty of ball kids hit, or that have fainted on court, or made a spectacular, unexpected catch. Though they’re tasked with blending in and not making themselves seen, sometimes they become part of the show.

In 2014, during a rain delay at the French Open, Djokovic invited a ball kid holding his umbrella to sit. He did. Djokovic then grabbed a bottle of sparkling water and the two toasted, the crowd roaring with laughter. The moment went viral online, with 12 million views.

“I usually try to be nice … I understand when there are some mistakes,” said Lucie Safarova, a finalist at the French OPen last year. As a player, “you’re usually very focused on the (match), so you're not very much like looking around yourself (to notice them).”

For Tennis Channel commentator Justin Gimelstob, he remembers it being hard to keep emotions in check during matches. “It’s a ‘do as I say, not as I did,’ situation,” he said.

“You have to treat everyone with the utmost respect. You’re dealing with a high-stress, high-intensity situation and sometimes your emotion gets the best of you. These ball kids are incredible. You’re dealing with different languages, misinterpretations, misunderstandings. It’s simple: Kids are so impressionable. You have to make sure that you are careful, very careful in how you treat them.”