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South Australia's Hughes had reached 63 not out when he was felled by New South Wales paceman Sean Abbott.
A statement from Cricket Australia doctor Peter Brukner on Tuesday night read:
"Phillip Hughes sustained a severe head injury while batting for South Australia at the SCG today.
"Phillip was treated on the ground by Cricket NSW medical staff and transferred by Ambulance to St Vincent’s Hospital.
"He subsequently underwent surgery and remains in a critical condition in the hospital’s intensive care unit.
"Unless there is any significant change there will be no further updates on his condition until tomorrow.
"Phillip is receiving the best possible medical care. The thoughts of his teammates and the wider cricket community are with Phillip and his family and friends at this difficult time.
"We ask that you kindly respect the family’s wishes for privacy.
The incident cast a pall over the SCG as ambulances rushed to the ground.
Phil Hughes staggered before collapsing face-down onto the pitch. Photo: Getty
He was attempting to play a pull shot, and the ball hit him flush.
After the ball cannoned into his helmet, Hughes stood dazed before crashing face-first onto the pitch.
MORE: Hughes injury reminiscent of Ewen Chatfield’s horror
The NSW players and umpires called for help immediately, and the tea break was called 30 minutes early.
A medical cart transported Hughes from the field.
The South Australian Cricket Association (SACA) released a statement saying Hughes's family accompanied him to the hospital along with SACA high-performance manager Tim Nielsen.
Both SACA and Cricket Australia (CA) extended their "thoughts and prayers" to Hughes and his family.
"His welfare is our highest priority," CA chief executive James Sutherland said.
"We're also naturally concerned about all of those involved in today's game and will be giving them our utmost support."
The left-hander is bidding for selection in Australia's first Test team which plays India in Brisbane next week.
He was considered a leading candidate to replace injured No.4 batsman Michael Clarke.