NEW YORK -- Novak Djokovic, the odds-on favorite to win his 18th Grand Slam title at this US Open, is gone from the tournament in the most sensational default recorded in the Open era.

Djokovic, the top-ranked player in the world, was ejected from the tournament after he inadvertently hit a line judge in the throat with a ball smacked in anger while he trailed fourth-round opponent Pablo Carreno Busta 6-5 in the first set.

What exactly is a default, and are there other notable occasions when it has been used? Here are some frequently asked questions.

What is a default?

A default occurs when a player is removed from a match or tournament for any number of violations spelled out in the Official Grand Slam Rule Book or ATP and WTA tours code-of-conduct regulations. It can kick in because of a single, significant incident for an off-court infraction (illegal betting) or during play, as was the case with the default that ended Djokovic's tournament. A default is also the automatic result when a player commits a third violation of the code during a match. Code violations usually involve things such as unsportsmanlike conduct, stalling, abuse of equipment or officials, coaching or use of foul language. Players who are defaulted are finished in the tournament, and even if they were leading an opponent when the default was issued, the score goes down in their record as a loss.

How common are defaults in tennis?

Defaults are relatively rare. Players rarely commit single acts that mandate default and are careful not to accumulate enough code violations in a match. There have been just five notable defaults issued since 1963.

What rule did Djokovic violate to earn the default?

The Serbian star violated Article III: Player On-Site Offenses, Section N of the Official Grand Slam Rule Book, which deals with "ball abuse." It reads: "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). ... For the purposes of this Rule, abuse of balls is defined as intentionally hitting a ball out of the enclosure of the court, hitting a ball dangerously or recklessly within the court or hitting a ball with negligent disregard of the consequences."

The USTA issued a statement not long after the default: "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 tournament referee defaulted Novak Djokovic from the 2020 US Open."

Who actually defaulted Djokovic?

In order to avoid poor or rash decisions, there is a procedure for adjudicating default-worthy incidents. In this case, chair umpire Aurelie Tourte summoned the US Open tournament referee, Soeren Friemel, and the chief Grand Slam supervisor, Andreas Egli. They consulted with one another, collected all the relevant facts and came to a decision.

Could Djokovic appeal the decision?

No. According to the Grand Slam record book, "In all cases of default, the decision of the referee in consultation with the Grand Slam Chief of Supervisors will be final and unappealable."

Does Djokovic's default come with other punishments?

Yes. The USTA said in its statement: "Because he was defaulted, Djokovic will lose all ranking points earned at the US Open and will be fined the prize money won at the tournament in addition to any or all fines levied with respect to the offending incident."

The blow to his rankings is somewhat alleviated by the temporary rule adopted by the ATP and WTA to mitigate the recent coronavirus lockdown. The rolling system on which the rankings are based has been extended from 12 to 22 months (March 2019 to December 2020). Under the system, if two editions of the same tournament take place during the rankings period, a player's best result is used in calculating his ranking. Djokovic will be able to retain the fourth-round rankings points he earned at the 2019 Open. The USTA has not yet announced the amount Djokovic will be fined, if at all, for the incident. He loses the $163,000 he earned for reaching the fourth round.

Does a default keep a player out of upcoming tournaments?

A default does not automatically bar a player from competing in other tournaments unless the player is also suspended. It's unlikely that Djokovic will have to sit out any tournaments in which he wishes to play.

Does a player need to cause bodily harm to be defaulted?

No. Goran Ivanisevic, a former Wimbledon champion and Djokovic's current coach, was defaulted from a tournament in 2000 at Brighton, England, for smashing all his rackets in a second-round match against Hyung-Taik Lee.

Have other players been defaulted in major tournaments?

There have been three high-profile defaults in Grand Slam tournaments since 1963, including Djokovic's.

In 1990, John McEnroe was defaulted from the fourth round at the Australian Open, largely because of his own miscalculation. He was unaware that the four-step process leading to a default for code violations had been reduced to a three-step process. His worst violation was abuse of equipment and officials.

In 1995, Tim Henman, the British player who had the reputation as a great sportsman, accidentally hit a ball girl in the side of the head during a Wimbledon match. Angry about losing a point, he smacked a ball toward the net unaware that the ball girl was running across the court.

Two other notable defaults since 1990 include David Nalbandian's ejection from the final of the Queens Club event in 2012. Nalbandian was leading Marin Cilic by a hair when he lost a critical service game and kicked an advertising board. The wooden sign shattered, and part of it flew into the leg of the nearby line judge, leaving a nasty gash.

The most recent default before Sunday was in a 2017 Davis Cup tie featuring Canada's Denis Shapovalov, who was 17 when he hit a ball in frustration and caught the chair umpire in the eye, breaking the umpire's eye socket. Shapovalov later said he was "incredibly ashamed and embarrassed."