It was a double match at Roland Garros and it happened in the second set when Kato sent a ball towards a ballgirl but she wasn't looking at the ball and got hit with it after failing to dodge it. It resulted in a code violation then and a default later after the supervisor came to the court and assess the situation.
According to Kyrgios the default was justified which makes sense as it is the rule. Plenty of people complained about it but it is according to the rules. Tennis folk in the know-how agree with Kyrgios, it was likely the right call no matter how heartbreaking it was. He also talked about the continued occurrence of players getting booed at Roland Garros.
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Kyrgios has some experience with it as he got booed plenty in his career. It's just part of the sport and every player has to deal with it at one point in their careers. It doesn't matter where you play, sometimes you get cheered on and other times you will get booed:
"all around the world, whether it be having people chanting your name, idolising you, getting booed at, it’s always going to be a factor in sport. Why are we having so many people complain about it? Also, it’s not about the intent, if you a hit a ball at a kid it’s a default."
Kyrgios should be making a return to tennis soon as the grass season is starting up again. Soon we'll see whether he gets booed or cheered upon though it's mostly the latter. Unless he does something questionable on the court. That is always a possibility.