The 25-year-old, who was pressing for a Test recall, was knocked out by a Sean Abbott bouncer in a freak incident at Sydney Cricket Ground on Tuesday, crumbling to the ground unconscious after the heavy blow to the lower head.
He underwent surgery and was in an induced coma in the St Vincent's Hospital intensive care unit.
"Phillip Hughes remains in a critical condition in St Vincent's Hospital after being struck in the head while batting for South Australia against NSW at the SCG yesterday," said Cricket Australia in a statement.
"He will have scans this morning and a further update will be provided when the results of those scans are known."
Test captain Michael Clarke was one of the first to arrive at the hospital on Wednesday, resuming a vigil at his close friend's bedside and comforting Hughes' mother and sister.
Hughes made his Test debut in South Africa in 2009, where he made 75 in the second innings at Johannesburg. The left-hander followed up in the second Test at Durban with centuries in both innings, amassing some 275 runs at the crease.
The runs dried up and despite playing 26 Tests he has never secured a regular place in the team, partly due to his perceived weakness against the short ball.
But with doubts over the fitness of Clarke for the first Test against India next month in Brisbane, he was seen as a potential replacement.
The popular Hughes was wearing a helmet when he was struck but manufacturer Masuri said he was not wearing their most up-to-date model.
"From the footage and pictures currently available to Masuri, it appears that Phil Hughes was struck by the ball to the rear of the grille and below the back of the shell, missing his Masuri Original Test model helmet," said the company .
"This is a vulnerable area of the head and neck that helmets cannot fully protect, while enabling batsmen to have full and proper movement."
Hughes was attempting a pull shot when he was hit, and fell badly.
Medical staff worked on him, including mouth-to-mouth resuscitation and oxygen, for more than 30 minutes before he was taken from the field and rushed to hospital.
Up-and-coming bowler Abbott, who made his one-day and Twenty20 debut for Australia in October, was seen cradling the injured player immediately after the incident.
"It says a lot about Sean doesn't it," said Cricket Australia chief James Sutherland , calling it a 'freak, freak accident'.
"Sean will have all the support he needs around him."
Cricketers from around the world have offered support to Hughes and many also rallied behind Abbott.
"Stay strong @seanabbott77. Not your fault young man," tweeted former Australian great Dean Jones, while Damien Fleming said: "Feeling for Sean Abbott who was just bowling a cricket ball."
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Get well soon Phil!
Not a sight we want to see on a cricket field. Do feel for Abbott also, it's all part of the game buddy!
Stay strong!