Former Test batsman Phil Hughes remains in a critical condition in hospital on Wednesday morning after a short-pitched delivery struck his skull during a Sheffield Shield match at the Sydney Cricket Ground.

Hughes, who was playing for South Australia against his old state NSW, was felled by the bouncer from fast bowler Sean Abbott at 2.23pm on Tuesday. The ball struck him on the left side of his head, behind his ear.

A St Vincent's Hospital spokesman said on Wednesday morning that Hughes' condition had not changed overnight. He remained in an induced coma in the hospital's intensive care unit.

Australian captain Michael Clarke was at the hospital with Hughes' family until late on Tuesday, and was seen returning to the hospital just before 7.30am on Wednesday.

Hughes collapsed just seconds after he was struck in the head by the cricket ball. The 25-year-old put his hands on his knees, looked towards his feet, and collapsed face-first onto the SCG pitch.

As Hughes collapsed, Abbott rushed to his side along with umpires and his former teammates, calling for urgent medical attention through NSW doctor John Orchard.

Dr Orchard administered CPR and paramedics intubated him and took him to hospital in a waiting ambulance.

He immediately underwent surgery after arriving at St Vincent's Hospital, with doctors working to relieve pressure to his brain.

Hospital staff said it would not be known how successful the procedure would be for at least 24 hours and Hughes was in a critical condition and in an induced coma.

At 9pm on Tuesday, Australian team doctor Dr Peter Brukner said Hughes remained in the hospital's intensive care unit.  

"Unless there is a change in condition there will be no further updates on his condition tonight," he said. 

"Phillip is in the best possible medical hands. The Cricket Australia community and his team mates have thoughts of thinking of his family and his friends at this moment."

Hughes was being considered by the national selectors to replace Clarke, who was likely to be ruled out of the team because of a hamstring injury for next week's first Test against India at the Gabba.

Hughes' mother and sister were cheering Hughes on at the ground when the incident occurred, and rushed to be by his side at the hospital.

Clarke and former NSW wicketkeeper Daniel Smith were among the dozens of players who arrived to support the Hughes family in the emergency department on Tuesday.

On Tuesday afternoon, the South Australian Cricket Association's Tim Neilson released a statement on behalf of Hughes' family.

"Phillip Hughes suffered an injury this afternoon. He is presently undergoing surgery and the outcome of that surgery is unlikely to be known for the next 24 to 48 hours.

"In the meantime we would ask, we would really appreciate it, if you would look after the family and the friends and respect their privacy."

In the minutes that followed the delivery, players including Australian opener David Warner held a white sheet to screen Hughes from the few hundred supporters in the SCG's Members and Ladies stands.

The first of three ambulances arrived at the SCG 15 minutes after Hughes was hit. A medivac helicopter landed on the playing surface but it was not used to transport the cricketer to hospital.

The match was abandoned out of respect for Hughes, who was not out on 63.

Cricket Australia chief executive James Sutherland extended his best wishes to Hughes, who has played 26 Tests for Australia. 

"We were obviously shocked to learn of the seriousness of the incident involving Phil," he said.

"While we have faith that he's receiving the best available treatment, the situation will impact his family, friends, and those directly involved on the field.

"The ACA is therefore working closely with CA to arrange the appropriate counselling for those affected.  Naturally our thoughts are with him and we wish him well from all the ACA members."

As news spread of his dire predicament, cricketers including Warner and the Australian coach Darren Lehmann joined fans in tweeting their support for the popular batsman.

Others also urged people to show support for the bowler Abbott, describing his delivery as routine.

SMH