What the 30-year-old did en route to an agonising five-set defeat in the third round by Roger Federer was illegal - according to the rules of the United States Tennis Association, which prohibit “any action that materially changes the condition of the ball; therefore, a player may not use a ball to wipe off perspiration.”

And was not illegal according to the International Tennis Federation rules used at the Australian Open, which make no mention of such actions.

Setting aside the legal status of Millman’s apparent wiping of the ball across his sweaty shirt, we do at least know that such an action will speed up the delivery of a serve.

This was the conclusion of the person who instigated the controversy with a tweet including a clip of the action, tennis coach and ex-player Sven Groeneveld.

"Millman applying the old trick in speeding up the first serve by rolling the ball on his (I assume) wet shirt before he serves? Is that legal in tennis I know it’s not in other ball sports like cricket and baseball? Do we have a rule in tennis?" Groeneveld wrote.

But Rob Koenig, a tennis commentator for ATP Tennis on Amazon Prime, replied to Groeneveld questioning whether a wet ball would travel faster.

He asked: “You sure it makes it faster and not slower?? [sic] A moist ball would feel 'heavier' and get 'bigger.' A drier ball (like we see in IW or Dubai) zips through the air, right?”

Britain’s former Davis Cup player and US Open finalist Greg Rusedski was drawn in at this point, although his comment - followed by a laughing emoji - was strangely ambiguous.

“I think the ball becomes faster as the fluff of the ball is compacted, unless you are putting it in water”, he mused.

Some voices were heard insisting that wet balls travel slower than dry ones, while another response insisted that wet balls skid when they land.

Another Twitter user, under the name 'tennisblog', backed up that suggestion: “Moisture on the ball makes it heavier so you can swing harder and the ball is more likely to go in because it won't fly. Millman said this is what he liked in NY as the humidity allowed him to swing for the fences. So I guess he was trying to recreate that last night.”

Perhaps that was the truth of it as the unseeded Aussie looked for every incremental advantage while locked into a match that appeared, tantalisingly, like it could produce the biggest win of his career against a living legend of the game.

And perhaps not.

Former Australian player Todd Woodbridge contributed his own valid view to the debate in pointing out that most players will carry a second ball in their shorts while serving the first, which may have a similar effect to contact with a sweaty shirt.

Groeneveld also took the time to reply to Koenig, pointing out that - in his eyes - the question is not faster or slower but does this alter the state and speed of the ball?

This is a different question, and appears to frame the incident more broadly, implying that it is a matter primarily of determining intent, regardless of the exact consequences of the action.

Grey area. There are many of them in life and in sport. It’s like spotting an ant; see one, soon you see loads.

So this debate also extended into other grey areas. Among Groeneveld’s former charges, for instance, was Maria Sharapova - a player who has been accused of a number of instances of gamesmanship, primarily the tactical use of toilet breaks and of excessive grunting when hitting the ball. 

Grunting is widely viewed as an off-putting tactic which some professionals say can be used to cover the sound of the return off the racket - something that ordinarily offers clues to the opponent.

Groeneveld was also Sharapova's coach when she served a 15-month ban for an anti-doping violation. 

Others got into sporting relativism by mentioning the amount of apparent interaction during the match between Millman and Australian player-turned-coach Lleyton Hewitt, who appeared to be sending a number of signals to him which were overlooked by the chair umpire.

Some would argue that this constituted more of an unfair advantage than serving balls with a light coating of sweat. Again, the position vis-à-vis the rules seems unclear. 

Coaching from the sidelines during matches is supposed to be illegal. It was certainly so when Serena Williams was dragged over the coals for communicating with coach Patrick Mouratoglou during the 2018 US Open final against Naomi Osaka. 

But freelance writer Simon Cambers has today broken a story for ESPN that the Women's Tennis Association is to allow coaching from the stands for a trial period.

As I say, quite clear.

But not absolutely clear.