A day after being controversially disqualified from the French Open for inadvertently hitting a ball girl with a ball, an emotional Miyu Kato struggled to answer questions about the incident.

Kato and her partner Aldila Sutjiadi were defaulted from the doubles event on Sunday after the Japanese player struck a ball girl having innocuously hit the ball to the back of the court following a point.

Kato, who was seen crying on court after her disqualification, was back in action on Monday and reached the mixed doubles semifinals after winning in straight sets alongside German partner Tim Puetz.

It's widely reported that Kato was in tears on court after winning the quarterfinal and was visibly emotional in the press conference afterward, leaving the room momentarily to compose herself before a question was answered.

"I think, first and foremost, we're all, including Miyu, happy that the ball girl is okay because obviously she got hit," Puetz told reporters after coming back into the room with Kato.

"Miyu feels terrible about what happened. This was just very unfortunate for everyone, especially for her and the girl."

A tearful Kato gave a very short answer in Japanese, saying it was difficult for her to speak English at this time.

While Puetz understood why officials made the decision, he said there was clearly no intent to hurt the ball girl.

"I understand she hit the ball girl. It was not intentional. I don't think it was in anger," he added.

"I think everybody saw the pictures. It wasn't anything malicious that she did. Nevertheless, it hit the ball girl.

"Then two supervisors have to come on court, possibly didn't even see it. All they see is a crying ball girl who got hit with a ball.

"In that moment, to make that decision, is very difficult."

On Monday, Kato tweeted an apology to the ball girl, her doubles partner and her supporters, saying the incident was "completely unintentional."

"I want to thank everyone for your continued support. I have received a worldwide outpouring of positive energy, and it has raised my spirits tremendously," she wrote, in a post which garnered support from other players.

"Now I'm onto the mixed doubles semifinals, where I will use every bit of that positive energy to succeed!"

The rules

According to the grand slam rule book, players "shall not violently, dangerously or with anger hit, kick or throw a tennis ball within the precincts of the tournament site except in the reasonable pursuit of a point during a match (including warm-up)"

The rule book states: "In all cases of default, the decision of the referee in consultation with the grand slam chief of supervisors shall be final and unappealable."

A player who is defaulted loses all ranking points earned at the tournament, the rule book states, and all prize money earned at the tournament.