Almost 40 years ago, John McEnroe screamed four words of fury at Wimbledon: "You cannot be serious!"

Not that he always felt it was to his benefit.

"I became what I would say was like a cigarette smoker that couldn't kick the habit," McEnroe said in a 2018 interview. "It felt like I was doing it for the wrong reasons."

One reason was entertainment.

With ATP and WTA events returning behind closed doors this month, a leading sports psychologist believes the lack of fans is likely to see fewer controversial outbursts.

"The expression of anger can be a strategic way of releasing frustration, but it can also be a communication function and a way of entertaining the fans," says Andy Lane, who has worked with elite athletes across various sports for over 30 years.

"When there is no crowd, you're not frustrated because you're losing in front of a crowd, you're not communicating to anyone other than your opponent, and you're not trying to entertain a crowd.

"Without the expectation of fans, it is like walking out to a training match. They are walking in cold without any noise to gee them up, so they are relaxed when they get on to court and they will go through their pre-set routines.

"That means angry outbursts will be less likely. If you do see any, they are more likely to be pre-planned because fans usually fuel these acts."

Why are people are entertained by anger?

Few things pump up a tennis crowd like seeing a raging player expending a whole load of negative energy.

Players get annoyed at many things - but mainly by their own failure to execute the shots they practise every day.

Another trigger is a perceived injustice by an official - like that which led to McEnroe's rant during his Wimbledon first-round match against Tom Gullikson in 1981 or, more recently, Serena Williams' outburst during the 2018 US Open final against Naomi Osaka.

This behaviour is not condoned by the tennis authorities, who can penalise the offenders competitively and financially.

Yet it can bring new eyes to the sport by providing an extra element of entertainment.

"Humans connect very well to emotions and anger - the red mist of losing control is something we all get," says Lane, a professor of sports psychology at the University of Wolverhampton.

"So when you see someone so good doing it you connect closely - and many enjoy watching it."

Some players become better known to a wider sporting audience for isolated outbursts rather than their on-court achievements.

For many, Argentine David Nalbandian is defined by kicking an advertising hoarding at Queen's in 2012, injuring a line judge. Russian Mikhail Youzhny cut his head during a 2008 tournament after hitting himself with his racquet.

More recently, clips of Czech former world number one Karolina Pliskova whacking a hole in an umpire's chair and then-ATP Finals champion Alexander Zverev destroying a racquet at the Australian Open were widely spread on digital platforms.

Racquet smashes become part of the entertainment. Marcos Baghdatis was egged on by a cheering Melbourne crowd when he demolished four in the space of a minute during an Australian Open defeat by Stan Wawrinka in 2012.

"It is child-like. It is relatable," says Lane. "For the players, it is a fine balance between squashing down the emotion and carrying the bad shots into the next game.

"Not many players lose their cool during a point, they lose it at the end of a game. It is a strategic way of refocusing.

"It tends to be a racquet smash because that's the only thing they can 'blame'; they don't have any team-mates to be angry with."

How audiences are still attracted to controversy

When McEnroe screamed 'You cannot be serious' at umpire Edward James after disputing a line call during that match against Gullikson, it became one of Wimbledon's most famous moments.

It has spawned a million punchlines, countless impressions and became the title of one of McEnroe's books.

The clip has received almost 1.5m YouTube views via the Wimbledon and ESPN channels in the past five years.

"McEnroe was a showbusiness player who used anger as a crowd puller," says Lane, also a consultant for the Centre for Health and Human Performance in London.

"I think Nick Kyrgios has got a bit of that in him, even though he might not admit it. And because of it, people have heard of him, whereas they won't have heard about players around him in the rankings."

That is supported by the digital data gathered by sports analytics platform Hookit.

Despite never being ranked inside the world's top 10 nor past a Grand Slam quarter-final, the Australian has 2.4m followers on social media platforms.

Only five players in the men's and women's top 10s have more - Rafael Nadal (39.8m), Roger Federer (35.3m), Williams (28.7m), Novak Djokovic (23.1m) and Simona Halep (3.6m).

In 2020, Kyrgios has more social engagement (more than 5.7m likes, comments, and shares) than everyone in the top 10s other than Djokovic (24.5m), Nadal (22.5m), Williams (17.2m) and Federer (14.9m).

As well as social media, this online interest also translates to the BBC Sport website and app, where stories involving Kyrgios attract sizeable audiences.

The post-match interview at Wimbledon 2019 where Kyrgios said he "wanted to hit" Nadal with a shot was seen by almost one million people in the UK alone, making it the website's fourth most-read tennis story of the fortnight.

Two months later, another controversial incident at the Cincinnati Masters - where Kyrgios smashed two racquets and called umpire Fergus Murphy a "potato" - attracted seven times the typical number of views for a tennis video on BBC Sport.

However, this year's US Open will be without the Australian after he withdrew from the tournament because of the coronavirus pandemic.

One of the website and app's biggest tennis stories in recent years was Williams wagging her finger at umpire Carlos Ramos while calling him a "liar" and a "thief" in the 2018 US Open final. The BBC Sport story was read by more than two million people in the UK.

The incident helped the match attract an average of 3.1m television viewers in the United States, more than a 50% hike on the following day's men's final, and resulted in headlines and analysis across the world from non-tennis media as well as tennis journalists.

How marketing fuels controversial moments

At Wimbledon in 1977, four years before McEnroe's infamous outburst, Nike founder Phil Knight was looking for a new "horse to back" in the tennis world.

American tennis officials warned him to stay away from McEnroe. Why? "Because he is a hothead," they said.

Knight described in his autobiography how he "fell madly in love" with the New Yorker and signed him up the following year.

"Nike has a long history of creating personas that are bigger than life, like LeBron James, Tiger Woods and Michael Jordan," says Kurt Badenhausen, a senior editor at Forbes magazine, who specialises in the business of sport.

"McEnroe was a wildly successful and charismatic player in the biggest market in the world.

"But he was also one of the biggest players of his generation because of the way Nike marketed him as the bad boy of tennis."

The current incumbent of the role is Kyrgios, whose major deals include Nike, Yonex and Beats By Dre.

According to Hookit's analysis, he has generated £310,000 of value for brands in 2020 with only Federer (£340,000) generating more.

"There is an argument right now that a lot of the players on the men's tour are indistinguishable to the casual fan, once you get past the big three and Andy Murray," says Badenhausen.

"A guy like Kyrgios stands out, he's edgy, he's walking that fine line and for Nike they can find a way that makes sense to use him.

"People recognise how talented he is, but how maddening he is. For Nike, if a guy like that can put it together and win Grand Slam titles, he is very marketable."