Novak Djokovic was defaulted in the fourth round of the 2020 US Open on Sunday after hitting a ball in frustration that struck a line judge on the court at Arthur Ashe Stadium. That line judge was hit in the throat and had to be helped off the court after appearing to have trouble breathing. 

Djokovic, a 17-time Grand Slam champion and top seed in this year's US Open, was disqualified from the tournament. Here are some looks at the incident:

The US Open released a statement shortly after Djokovic was defaulted:

In accordance with the Grand Slam rulebook, following his actions of intentionally hitting a ball dangerously or recklessly within the court, or hitting a ball with negligent disregard of the consequences, the US Open referee defaulted Novak Djokovic from the 2020 US Open. Because he was defaulted, Djokovic will lose all ranking points earned at the US Open and will be fined prize money won at the tournament in addition to any or all fines levied with respect to the offending incident. 

The default came after several minutes where Djokovic pleaded his case to tournament officials that he did not intend to hit the line judge. It was the second time he hit a ball in frustration after a point in the first set of his match against Pablo Carreño Busta, and this one came after Busta broke his serve at 5-5. Carreño Busta now moves on to the quarterfinals.

After the match, Carreno Busta told reporters he was stunned by what he saw. "I was in shock, I never expected this moment playing against Novak, he said. "When they [were] talking [at] the net, I just tried to be focused in case we had to continue playing ... I needed to be ready in case we had to continue." 

Hours after the incident, Djokovic took to Instagram to apologize to the line judge and the US Open with a statement.

This whole situation has left me really sad and empty. I checked on the lines person and the tournament told me that thank God she is feeling ok. I'm extremely sorry to have caused her such stress. So unintended. So wrong. I'm not disclosing her name to respect her privacy. As for the disqualification, I need to go back within and work on my disappointment and turn this all into a lesson for my growth and evolution as a player and human being. I apologize to the @usopen tournament and everyone associated for my behavior. I'm very grateful to my team and family for being my rock support, and my fans for always being there with me. Thank you and I'm so sorry.  

According to tennis writer Ben Rothenberg, Djokovic tried to downplay the incident on the court, reportedly saying "she doesn't have to go to the hospital for this," before he eventually left the USTA Billie Jean King National Tennis Center. 

Djokovic is known to be a demonstrative player on the court in moments of frustration, and when asked about those moments at the 2016 ATP Tour Finals, the World's No. 1 player was defensive.

According to the USTA Officiating: Scenarios and Interpretations guide, Djokovic's actions fall under the category of "Ball Abuse," which includes the following situations:

Deliberately hitting, throwing, or kicking a ball that is not in play that hits a person

Deliberately hitting, throwing, or kicking a ball that is not in play that leaves the playing area   

Deliberately hitting, throwing, or kicking a ball that is not in play that comes close to hitting a person 

Deliberately hitting, throwing, or kicking a ball that is not in play in a manner that could cause damage or harm  

With Djokovic out of the tournament, the US Open will now have a first-time Grand Slam winner. Furthermore, a men's Grand Slam will have a winner that isn't Djokovic, Roger Federer or Rafael Nadal for the first time since Stan Wawrinka won the 2016 US Open.