Australian cricket great Damien Martyn 'in induced coma'
The 54-year-old, who played 67 Tests between 1992 and 2006 and 208 one-day internationals, was admitted on Boxing Day last week after falling ill while lying down.
Australian media said he was being treated for meningitis, which can be deadly.
Meningitis is an inflammation of the membranes surrounding the brain and spinal cord.
"He is getting the best of treatment," Test great and close friend Adam Gilchrist told The Australian newspaper.
"Amanda (his partner) and his family know that a lot of people are sending their prayers and best wishes."
Former Australian coach Darren Lehmann, a long-time teammate of Martyn's, also sent support.
"Lots of love and prayers... keep strong and fighting, legend. Love to the family," he wrote on X.
Known as one of the game's great strokemakers, Martyn became a key figure in Steve Waugh's all-powerful Australian side, hitting 13 centuries and averaging 46.37.
He was also part of the Australia team that won the 2003 ODI World Cup, cracking 88 not-out in the final against India in a match-winning partnership with Ricky Ponting.
He retired in 2006 during the Ashes series and had kept a low-profile since.