UPDATE: Muhammad Ali died Friday at age 74, a family spokesman confirmed to NBC News.
Muhammad Ali is in "grave condition" in a Phoenix-area hospital, a well-informed source tells NBC News, as the boxing legend continues to battle a respiratory condition.
The 74-year-old former heavyweight champion's family has gathered by his bedside just a day after he had been hospitalized in what spokesman Bob Gunnell said Thursday was to be a "brief" stay.
A spokeswoman for his daughter, Laila, said, "Laila's number one priority is her father's well-being. She truly appreciates the outpouring of love for her family, as she spends quality time with her dad."
Ali has been battling advanced Parkinson's disease in recent years — an illness that experts say can lead to complications and affect the breathing of patients.
The boxing legend was previously hospitalized for several days in December 2014 and January 2015 after suffering a severe urinary tract infection initially thought to be pneumonia.
Although weakened by illness, Ali has continued to loom large on national stage.
In recent months, Ali waded into the nation's political fight after GOP presidential candidate Donald Trump suggested banning all Muslims from entering the U.S.
Ali, who famously joined the Nation of Islam in 1964, asked for more understanding.
"Speaking as someone who has never been accused of political correctness, I believe that our political leaders should use their position to bring understanding about the religion of Islam and clarify that these misguided murderers have perverted people's views on what Islam really is," Ali said in a statement.
But Ali has rarely been seen in public since last September, when he attended an awards ceremony by the University of Louisville honoring his prolific boxing career and legacy as a hometown hero. The Muhammad Ali Childhood Home Museum in Louisville opened earlier this month.