The 26-year-old, who was ranked as high as 13, is set to exit the top 100 following his withdrawal from the ATP tournament in Melbourne this week due to illness.

He was last that low back in 2014 when he was a teenager climbing the rankings after upsetting Rafael Nadal en route to the Wimbledon quarter-finals.

Making a slow recovery from a respiratory illness, Kygrios will contest next week's Sydney International - his only match practice ahead of the Australian Open which starts on January 17.

Kygrios, who has suffered asthma since childhood, said he was unable to get out of bed for almost four days.

"With everything floating around these days, I thought it was COVID - I was feeling pretty average," the Canberra ace said.

"I was testing a lot but it came back negative.

"I was bedridden and I'd hit for half an hour and I was out of breath. I felt lethargic, I felt sleepy.

"I still am feeling some effects like my breathing is a little bit affected and I don't feel 100 per cent on court as yet, but I'm just slowly getting better every day."

Kyrgios didn't want to play and be criticised for a below-par showing, although he said he was in the firing line anyway.

"I don't actually think I was able to compete fully, like I was scared that I'd go on court and feel tired and lethargic and I didn't want people thinking all like 'he's not trying'.

"Even when I pulled out, I dealt with ridiculous amount of hate comments and stuff and 'it's like bro, I'm just like sick'."

After overcoming a knee injury following rehab and training in Canberra, Kyrgios is in a happy place mentally.

He feels he still is a major danger in any tournament.

"Honestly, if I'm ranked 1000 or 10 in the world I know what I'm capable of and everyone knows what I'm capable of on Tour," he said.

"I'm not a player that hasn't proven themselves - I talk a lot but I also have beaten a lot of players and I've won a lot of tournaments so that's not something I'm focusing on," he said.

"I just want to go out there have fun and I want to put on a good performance.

"It's the Australian summer and people expect me to put on a good show and I think that I'm still capable of doing that.

"I won Acapulco unseeded and I beat four top-10 players ... I think the level of tennis has never been this deep, everyone can play, everyone's capable of doing very, very well."

Australian Associated Press