Kyrgios, 30, has played just five singles matches in the last three years because of multiple surgeries on wrist and then knee injuries.
"In the last month, I don't know what it is. I was with my masseuse and physio last night and something really has changed with my knee," the 2022 Wimbledon finalist Kyrgios told the Australian Associated Press.
"It's not swelling. It's not feeling bad after a session.
"I don't know whether to call it a miracle or anything, but my knee feels like it's gotten younger by a couple of years."
Kyrgios said he could not explain why, after months of struggling with rehabilitation, there had been such an unexpected and rapid improvement.
"I don't want to jinx it, but something in it has gone, kind of allowed me to put three, four days of training together on court for an hour-and-a-half, two hours and be able to actually recover and then build on that," said Kyrgios
"So that's been really exciting and I haven't really told anyone. I have a new lease on life on court."
Kyrgios, whose ranking has slid to 652 in the world because of his inactivity, would need a wild card from organizers for the Australian Open.
He has not played since losing in the second round of the Miami Open in March.
Kyrgios is due to face women's world number one Aryna Sabalenka on December 28 in a rare "Battle of the Sexes" exhibition match in Dubai, organizers said this week..