Two unidentified sources familiar with his condition have told the Associated Press that he may be suffering more serious problems than his previous hospital stays.
And another has spoken to the Radar Online website, painting a grim picture for the 74-year-old boxing great.
"Ali is in dire straits and is rapidly deteriorating," the source told Radar.
"His breathing has become very shallow, requiring tubes."
The legendary boxer is fighting respiratory issues that are complicated by the Parkinson's that he was diagnosed with in the 1980s.
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However, a spokesman for Ali, Bob Gunnell, said there is no update on his condition from Thursday, when Ali was reported to be in fair condition, needing only a brief hospital stay.
Ali has been hospitalised several times in recent years, most recently in early 2015 when he was treated for a severe urinary tract infection.
He has suffered from Parkinson's for three decades, most famously trembling badly while lighting the Olympic torch in 1996 in Atlanta.
Doctors say the Parkinson's likely was caused by the thousands of punches Ali took during a career in which he travelled the world for big fights.
News of his hospitalisation brought well wishes from boxers and others on Twitter, including Sugar Ray Leonard, who modelled his career after Ali's.
"Prayers & blessings to my idol, my friend, & without question, the Greatest of All Time MuhammadAli ! #GOAT," Leonard wrote.
