YOU can tell a lot from a tennis player's face. Especially these days, when camera super slo-mos are so revealing that they capture the life cycle of a bead of sweat with the precision of a David Attenborough nature programme.

Not only that, you see everything else up close and personal, too. You know when they know they've missed a simple forehand winner down the line because their frustration and anger is that of teenage road rager. You know when they're buoyant because you get that arrogant look of the city trader about them; then conversely there is the thousand-yard stare of the soon-to-be vanquished. In its rawest form, tennis is a brutal, lonely sport, and the face provides a window to the psychological battle raging within.

It's physiological, too. You can see the pained expression whether brought on by over-exertion or physical injury. Basically, there is no hiding place.

So when Novak Djokovic thwacked a ball away in frustration during his last-16 tie against Pablo Carreno Busta at Flushing Meadows on Sunday night, it was possible to see the exact moment the world No.1 knew that his US Open was over. There was that look of horror as his hands went up to his face, he knew this was a car crash and the rest of have known for some time that it has been coming for a while.

The Serb is well aware of the regulations. Just as he knows that you change ends after every seven games or that Grand Slam matches are played over the best of five sets, punishments for dealing with unsportsmanlike conduct are specifically outlined in black and white in the tournament rule book.

Yet he still stood on court arguing his case with referee Soeren Friemel, suggesting that because the line judge did not require hospital treatment clemency should have been shown.

But there was a precedent here. Tournament officials frown on any dissent that endangers the safety of spectators, referees, ball boys and ball girls, opponents or any other person who finds themselves in the line of fire and is subsequently fired upon.

Tim Henman was infamously expelled from the Wimbledon men's doubles tournament with partner Jeremy Bates in 1995 for hitting a ball girl with a ball while David Nalbandian was disqualified from Queens in 2012 for a staggering act of petulance when he kicked a line judge on the shin. More recently Canada's Denis Shapovalov forfeited a Davis Cup rubber for smashing a ball into the ground which hit the umpire in the face.

Djokovic has sailed close to the wind on previous occasions. Indeed, even in Sunday's ill-fated match against Carreno Busta he was exhibiting signs of frustration but this was a re-run of previous near misses.

At the ATP Tour finals in 2016, he hit a ball into the crowd and then acted with incredulity when asked about it later by Neil McLeman, The Times sports journalist, dismissing suggestions that this was problematic behaviour for a player at the top of the sport.

What's more, it followed an incident in the same year against Tomas Berdych at the French Open during which Djokovic had thrown his racket and just missed a line judge. Last year, in the Monte Carlo Open, he slammed his racket into the clay and threw it into the crowd during an epic meltdown.

In the aftermath of Sunday's transgression, he skipped the mandatory press conference following the match and incurred a $20,000 fine in the process, not to mention the $126,000 he has now lost in prize money.

It was a bad look, the more so since Shapovalov, a 17-year-old at the time of his incident in 2017, immediately apologised on camera when conducting the post-match debrief. This was a teenager owning his error; Djokovic is 33 and the self-appointed head of the newly-formed Professional Tennis Players Association.

But with Djokovic there has always been a sense of entitlement. Witness his decision to organise a tournament in Belgrade at the height of the coronavirus pandemic which led to positive Covid-19 results for two of the participants.

On Sunday on social media, the fanboys and fangirls leapt to his defence claiming – as Djokovic did himself – that the punishment did not fit the crime and that there were a range of other sanctions available to Friemel.

Djokovic could have pointed to Aljaz Bedene, who was given a mere code violation for hitting a cameraman, at the recent Western & Southern Open. The Slovenian was lucky to escape but even so this was different. As Djokovic noted this was “centre stage” at a grand slam and because of that and the dramatic reaction of the line judge, his fate was sealed. Yes, the severity with which he hit the ball was on the slower side but that does not mitigate what he did and, at the risk of repeating oneself, he knew it.

If it had been an isolated incident then perhaps he might have had a point but this wasn't: Djokovic has just been lucky in the past.

What's more, when you've got John McEnroe, one of the most-volatile players the game has ever seen, calling you out for a rookie mistake – and you've spurned a glorious chance at an 18th grand slam – you know you've messed up.