Kyrgios pulled out of the Sydney Tennis Classic on Monday just hours before his match against Italian firebrand Fabio Fognini.

The 26-year-old withdrew from the Melbourne Summer Set last week due to a mystery illness that sparked his asthma.

Kyrgios was cleared of COVID-19 at the time, but on Monday confirmed he had contracted the virus.

"I just want to be open and transparent with everyone, the reason I have had to pull out of Sydney is because I tested positive for COVID," Kyrgios posted on his Instagram story.

"I am feeling healthy at the moment with no symptoms.

"I wish everyone all the best and to stay safe where you can. If all goes well I will see you all at the Australian Open."

The Australian Open begins on January 17, giving Kyrgios just a week to recover and be cleared to return.

Monday's COVID-19 diagnosis came as a shock after Kyrgios had undergone numerous tests last week after being hit by a mystery illness.

All of those tests cleared Kyrgios of COVID-19, but he still decided to withdraw from the Melbourne Summer Set to give his body more time to recover.

"Whatever I've got at the moment is really wearing me down, kind of been bedridden for the last couple of days," Kyrgios said at the time.

"Really struggling to breathe, my asthma has been acting up like crazy.

"I have had several COVID-19 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."

Despite his ranking slipping to 114, Kyrgios remains one of the biggest drawcards in world tennis due to his amazing abilities and knack for on-court meltdowns.

Kyrgios made the quarter-finals of the Australian Open in 2015, and has reached the fourth round on two other occasions.

But even if Kyrgios does beat the odds to earn a start at this year's Australian Open, he faces a monumental task to cause any type of damage.

Kyrgios hasn't played since a straight sets loss to world No.4 Stefanos Tsitsipas at the Laver Cup in September.

It means Kyrgios will have gone almost four months without a competitive match if he recovers in time to play at Melbourne Park.

Kyrgios has proven in the past he has the ability to hit the ground running after extended breaks, but his recent illness plus a COVID-19 diagnosis adds a whole new level to the challenge confronting him.

Meanwhile, Australian Jordan Thompson has booked a spot in the second round of the Sydney Tennis Classic with a 6-4 6-2 win over American Marcos Giron.

The result sets up a showdown with world No.25 Reilly Opelka on Tuesday in what will be a big test for Thompson ahead of the Australian Open.

Australia's world No.173 Chris O'Connell, who has received a wildcard into the Australian Open, earned a spot in the main draw of the Sydney Tennis Classic with a 6-3 6-4 qualifying win over Italian world No.86 Stefano Travaglia.

Australian Associated Press