Novak Djokovic and other sports stars losing their cool

Novak Djokovic has been disqualified from the US Open after hitting a line judge with a ball.

The world number one had just lost a crucial point. In frustration he took a ball out of his pocket, and hit it behind him without looking, striking the judge in her neck.

"I'm extremely sorry to have caused her such stress. So unintended. So wrong," Djokovic said.

"This whole situation has left me really sad and empty.

"I checked on the linesperson and the tournament told me that thank God she is feeling OK."

Djokovic has apologised and said the situation left him feeling 'sad' and 'empty'

Djokovic, who was favourite to win the US Open tournament will lose all ranking points and won't be given any prize money he had won at the tournament. He could also be fined.

It also means the Serb will have to wait a little longer to win an 18th Grand Slam title. He's currently one behind Rafael Nadal and two behind Roger Federer on the men's all-time list of winners.

"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," Djokovic added in a statement on Instagram.

"I apologise to the US Open tournament and everyone associated for my behaviour. 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."

But Djokovic isn't the only tennis player who has got in trouble over the years.

Nick Kyrgios says if he had done the same thing as Djokovic, he would have got in more trouble

Nick Kyrgios

Australian Nick Kyrgios has a reputation for bad behaviour and suggested on Twitter that had he been in Djokovic's place, he would be banned for several years.

Kyrgios has has plenty of controversial moments; last year he hit Rafael Nadal with a ball at Wimbledon and after losing the match said he didn't need to apologise.

Nadal said that Kyrgios' actions were "dangerous", even though he did manage to get his racquet in the way of the ball.

But there is a history between the two players.

As a teenager, Kyrgios pulled off a shock win against Nadal, and said the Spaniard was "super salty" about it.

Serena Williams

Two years ago at the US Open final, Serena Williams had a massive row with the umpire.

Williams was given several penalties for breaking rules, including receiving signals and tactics from her coach in the stands - which isn't allowed.

At the time she told the umpire she didn't see any signals and that "I don't cheat to win. I'd rather lose."

During the second set Serena Williams smashed her racket into the court after she lost a game.

That's called 'racket abuse' and earned Serena another code violation.

Together with the first violation it meant a point was deducted.

The umpire also took a whole game from Serena, saying her outburst was 'verbal abuse'.

Williams accused the umpire of treating her unfairly because she's a woman. She said that if she was a man, speaking to the umpire in this way would never have be punished by a game penalty.

Serena Williams went on to lose the match to Naomi Osaka.

This US Open final didn't turn out exactly how either Naomi Osaka or Serena Williams had hoped

John McEnroe

He is one of the greatest bad boys of tennis. John McEnroe was always getting into trouble, losing points and getting fined.

One of the most famous on-court tantrums from McEnroe came in 1981.

In a first round match against Tom Gullikson, McEnroe was so angry at the umpire for not allowing his serve, he screamed: "You cannot be serious!"

After his tantrum, McEnroe went on to win the tournament, but his famous words in that match have been remembered forever, they even made it into pop songs and films.

But, it's not just tennis players who lose their cool. Check this lot out!

Luis Suarez got in trouble for biting, not once, but twice!

Luis Suarez

Barcelona, Liverpool and Ajax fans will tell you that footballer Luis Suarez has the ability on the pitch, but there has always been question marks over his behaviour.

Suarez's bad boy reputation started at the World Cup in 2010. In a quarter-final match against Ghana, the striker handled the ball to save a last minute Ghana goal. Suarez was sent off, but Ghana missed the resulting penalty and the African team went on to lose the match.

In 2013, Luis Suárez was handed a 10-game ban for biting Chelsea's Branislav Ivanovic, but he didn't learn his lesson, because at the World Cup a year later, her was biting again.

This time Suarez bit Giorgio Chiellini in a game between Uruguay and Italy. Suarez was given a nine-game ban from internationals, as well as a four-month ban from all football-related activity.

"I am going to have the same attitude. I will still run, still pressure, still argue," said Suarez. "No-one is going to change the way I play."

Russell Westbrook, herewith his old side Oklahoma City Thunder, is known for his high energy and argumentative behaviour on court

Russell Westbrook

NBA star Russell Westbrook has a reputation for winding up other players, referees and even fans.

So if you ever get the chance to watch him play basketball live, don't shout anything, because Westbrook often calls out fans who are cheeky to him.

Some fans think his outlandish behaviour often brings out his best performances on court, but referees might not always agree.