"Four weeks - four different balls. When are tournaments going to listen to players??"

Nick Kyrgios sympathises, with the Australian speculating that Novak Djokovic's current wrist and forearm issue might be down to changes in the balls.

"Changes of balls every week finally got to Novak's wrist," posted Kyrgios, the 2022 Wimbledon runner-up, after the world number one had treatment on court in Perth this week.

"The ATP really need to do something about the problem. Players suffer all the time."

Kyrgios added: "The people who think balls aren't a big enough factor to result in an athlete being hurt are potatoes. The load through a player's elbow, wrist over this vigorous season is enormous."

Tennis balls, made by competing manufacturers, are far from identical and with tournaments free to sign their own contracts, players are forced to chop and change throughout the season.

Fluffy balls which get increasingly heavy and slow - and seem to be increasingly popular - are the worst culprits.

The consequence, players believe, is more frequent injury - whether to the shoulder of Stefanos Tsitsipas, the elbow of Vasek Pospisil, the arm of Marketa Vondrousova or the wrist of Taylor Fritz.

Rafael Nadal spoke this month of hitting a "dead" ball in Brisbane.

But there is a chink of light on the horizon, as the men's ATP Tour is keen to centralise the system so it has more control over how a ball feels and plays during a certain stretch of the season.

There have also been discussions about whether the balls should be changed more frequently - perhaps every seven games rather than every nine - but this will need the agreement of the players, who have very different ideas about the ideal speed for a ball.

Players blame balls for injuries

Canadian Pospisil - a 2014 Wimbledon men's doubles champion alongside Jack Sock - has a dispiriting recent injury record and says he has "no doubts" the balls have been a contributing cause over the past two years.

He said a tear in the UCL tendon in his elbow last year had come after deep runs at tournaments "in a couple of weeks where all the players were complaining about how hard and heavy the balls were".

"It does take a toll," the 33-year-old told BBC Sport at November's Davis Cup Finals. "Tennis is a huge business and it would be good to be treated as partners in that business."

Pospisil, who was a co-founder of the Professional Tennis Players' Association, added: "I think player health is extremely important and it doesn't do the sport any favours if you have a lot of the top guys being injured."

The ATP says injury levels are consistent with previous years and there has not been a "huge spike" in injuries in any particular part of the body.

"The match lengths have extended significantly the last five to 10 years - 20-plus minutes per match on average - and that can be due to a number of factors," according to the ATP's chief tour officer Ross Hutchins.

"It's partly that the balls have created longer rallies, but that's also because of the strings players use, the way players hit the ball, and the physicality of the way they can retrieve balls is remarkable: it's all adding to longer matches.

"It means that if we don't adapt the balls, or how we approach our ball strategy, the rallies are going to keep getting longer, and the matches are going to keep getting longer, which is something that I don't think is our desire to have as an outcome.

"We haven't put any sort of direction, desire or mandate about trying to have longer rallies and we have no intention to do so."

American top-10 player Taylor Fritz ascribed the wrist issues he experienced in the closing months of last season to ball changes, saying he used three different types in three weeks.

Wawrinka was able to go one better, and highlighted the four balls he had to play with in four consecutive weeks on tour in Shanghai, Stockholm, Basel and Paris.

Russian 2021 US Open champion Medvedev said he had a "very big pain" in his wrist after his third-round match with Sebastian Korda at last year's Australian Open. He said hitting one particular ball felt like hitting an apple, the more it aged.

The same Dunlop ball is used consistently throughout January's Australian swing, and I am told the ball was fundamentally the same last year as it had been previously. But it was nonetheless unpopular with some around this time 12 months ago.

"I think that these balls are not good for hard courts. They get very fluffy, and as I say, it's a big shock to play them with your racquet," Medvedev said at the Qatar Open, which came three weeks after the Australian Open.

The durability of the ball is the priority and it will be interesting to see whether the 2024 model is more to Medvedev's liking.

Players on the WTA Tour have voiced fewer concerns, but there were grumbles from some at the US Open where - ironically, and after lobbying from players including world number one Iga Swiatek - women used the same heavy-duty balls as the men for the first time.

"They are very heavy and the season is long," Wimbledon champion Vondrousova said succinctly.

Why do tournaments use different balls?

Different surfaces demand a different type of ball, and temperature and moisture are also significant factors: a ball used indoors in Turin at the ATP Finals will react differently if hit by the same players in the dry heat of Melbourne at the Australian Open.

But there is no strong playing reason why a Penn ball would be used in Indian Wells and a Dunlop ball at the next outdoor hard court event in Miami just a few days later.

Or why a Wilson ball should be used at the French Open when a Dunlop ball has been used for the majority of the clay-court season.

Dunlop, which BBC Sport has approached for comment, is the official ball of the ATP Tour and will remain so until at least 2028 in a deal announced in November.

Nearly half of ATP events use the Dunlop ball, but as long as an alternative is approved at least 90 days before the start, individual tournament owners have plenty of opportunity to strike their own commercial deals.

And often they will feel they need to, just to help balance the books.

Is change on the way?

Nadal brought up the topic without being prompted after his victory over Jason Kubler at the Brisbane International.

"To be honest, the ball in that end of the first set was super big," he said.

"It was difficult to move the ball the proper way. I think the ball gets too big sometimes, especially under these humid and night conditions. I don't know what's going on with the ball, but it's dead."

Pospisil - having spoken about playing with the "heaviest balls in history" and the "facade of collaboration" - says this year is the first of his career where he has designed his schedule around the balls in use, rather than geographical location or court surface.

"In an ideal world you would still get the individual sponsorships by certain tennis ball companies," he said.

"But have a standardised ball and then print the logo particular to that company - making sure if you're playing on a grass court it's one ball, playing on hard court it's one ball, and clay courts [it's another]."

The ATP considered this option but prefers to retain a collaborative relationship with the manufacturers, who will nonetheless have to work within narrower specifications in future.

Tournaments are likely to have fewer balls to choose from: so even if the brand varies, the balls used in the South American clay-court season, for example, should all feel similar and behave accordingly.

With multi-year contracts in place at some events, it may take a while before the system is fully centralised, but the ATP's desire to act is shared by the WTA.

Changing the balls more frequently in matches and having more in play at any one time would keep them livelier and lighter for longer. That might reduce injury and match times but would disadvantage players who like nothing better than belting slow, heavy balls over the net.

This means players may demand significant notice of any change, and consensus may be hard to reach.