Nick Kyrgios has revealed he’s been battling a mystery illness after withdrawing from the Australian Open men’s warm-up tournament in Melbourne.

The Australian fan favourite was due to take on Slovakian Alex Molcan at Melbourne Park on Tuesday night in his return to competitive tennis following knee treatment late last year.

His late scratching will send the one-time world No.13 plummeting to around 115th in next week’s rankings.

It’s also huge setback to the hopes of 26-year-old Kyrgios topping his previous best Australian Open performance which came in 2015 — the last of only two grand slam quarter-final appearances.

Depending on his results in the Sydney Tennis Classic next week, where he has been granted a wildcard, Kyrgios may go into his ninth Open with just one match under his belt.

Kyrgios says he’s undergone multiple COVID-19 tests but they have come back negative.

The official reason for his withdrawal was asthma.

“I’m really sorry that I’ve had to pull out from this Melbourne Summer Set event,” he said in a statement released by Australian Open organisers.

“I have been feeling run down and unwell for four days.

“I have had several COVID tests over the last few days which all came back negative.

“I don’t feel 100 per cent so I need to take this week to be ready for Sydney next week,” he said.

Kyrgios, who has produced some of his best tennis in Melbourne, reached the third round of last year’s Open before blowing a two-set lead against former US Open champion Dominic Thiem.

The enigmatic star has been rehabbing his knee and training in Canberra in recent weeks.

He has been replaced by a lucky loser in the main draw of this week’s ATP tournament.

Andy Murray has endured a frustrating start to his season, losing in the first round of the Melbourne Summer Set to world No.76 Facundo Bagnis.

The former world No.1 and three-time major winner was far from his best, struggling on serve in an error-strewn 6-3 5-7 6-3 loss to the Argentinian.

Murray needed a wildcard into the Australian Open, three years after indicating his appearance in Melbourne was likely to be his last due to impending hip surgery.

-AAP