Nick Kyrgios, battling injury, loses his cool, smashes racket and hurls it into empty stand

Nick Kyrgios admits there is a "massive question mark" hanging over his Australian Open credentials as he battles a knee injury and ongoing mental hurdles leading into the grand slam.

Kyrgios was up to his usual tricks in a straight-sets defeat to Croatia's Borna Coric in a warm-up event at Melbourne Park on Friday, mixing some brilliant play with a mid-match dummy-spit.

The 25-year-old firebrand was warned by the chair umpire during the second set of his 6-3 6-4 defeat, after smashing a racket and hurling it into the largely empty grandstand.

He also vented his frustration over the blustery conditions on the outside court and ongoing pain in his troublesome knee, which he said flared up last year and has not fully settled.

"There's a massive question mark for me, even if I was completely healthy, [after] not playing in a year," Kyrgios said.

"It's not easy to just turn around and prepare for an event just like that. It's not like a tap.

"I thought I did everything I could this week. I won a couple of matches, I had a lot of court time and practised every day, as well.

"I think I'm ticking the boxes, I think I'm giving myself a chance."

Friday's result, which brought Kyrgios' Open preparation to a halt, came less than 48 hours after he stormed off the court and called the chair umpire a "smarta..." during his win over Harry Bourchier.

Kyrgios said his mental state is more important than anything and that he will use a few days off to refresh before his Open first-round match.

"If I'm positive and motivated, I'm sure I'll play some good tennis," Kyrgios said.