They have already teamed up off court though, Djokovic backing up Kygrios's initial jabs in a lengthy address of current world No.1 Sinner's ongoing doping case.

The International Tennis Integrity Agency (ITIA) levelled charges against Sinner and former women's world No.1 Iga Swiatek for breaches.

Sinner tested positive twice for an anabolic steroid in March, but avoided a ban because the ITIA determined he was not to blame - a decision the World Anti-Doping Agency has since appealed.

Swiatek also managed to accept a one-month suspension in November for testing positive to the banned substance trimetazidine.

Meanwhile Australian Max Purcell is serving a suspension for unknowingly receiving an IV infusion of vitamins above the allowed limit of 100 millilitres.

English doubles player Tara Moore served a 19-month ban that ended earlier this year for a doping offence she was ultimately cleared of.

Kyrgios has labelled Sinner and Swiatek's cases "disgusting" for tennis and described his sport's integrity as "awful".

Djokovic, who won Olympic gold but no ATP titles in 2024, was more tempered in his assessment.

“I’m not questioning whether (Sinner) took the banned substance intentionally or not,” he said on Sunday. 

"We've had plenty of players in the past and currently under suspension for not even testing positive to the banned substances. 

"Some players with lower rankings waiting for their case to be resolved for over a year.

"I've been really frustrated ... to see we've been kept in the dark for at least five months (on the Sinner case).

"The ATP hasn't really talked in depth about it. Why have they kept that case away from the public? 

"We see Simona Halep's case on the WTA Tour, now Iga Swiatek's case.

"It's not a good image for our sport. 

"I'm just questioning the way the system works and why certain players aren't treated the same as others.

"Maybe some ranking reasons are behind it or some players have more financial backing and stronger legal teams to tackle these cases."

Djokovic had knee surgery this year but returned to make the Wimbledon final, then opted out of the end-of-year ATP Finals as Sinner ended a year of dominance with an eighth title.

But the Serbian, a 10-time winner at Melbourne Park, thinks he still has years left in the tank.

He spent 10 days training with former foe Andy Murray before arriving in Australia and will have the Scottish great in his coach's box at the Australian Open before deciding if they continue beyond that.

“It’s strange for me to share all these kinds of insights about how I feel on the court, some of the secrets of what I’m going through, what I’m thinking about, how I see my game, with somebody who has been one of my top rivals," Djokovic said.

“But I’m so glad and very thankful that he has accepted to work with me, and in Australia … he’s very meticulous, dedicated and professional.”

Meanwhile, Australian wildcard Aleksandar Vukic sealed a 6-2 6-3 win over former world No.7 David Goffin in Sunday's only main draw match at the Brisbane International.

Vukic eased through the opening set by converting both his break points.

The 28-year-old from Sydney then broke again in the eighth game of a tight second set to wrap up victory in 82 minutes with a strong hold of serve.

Vukic next faces a potential meeting with defending champion Grigor Dimitrov if the Bulgarian can overcome qualifier Yannick Hanfmann.

"I think I definitely served good when I needed to. There were patches where I wasn’t making enough and getting into slight trouble on my serve and then when I needed to I served well … I thought my performance was very good,” Vukic said.

"It was my first year playing day in, day out at this level, a lot of ups and downs but really very confident going into the new season."

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