Dylan Jessen, 21, pleaded guilty in Adelaide Magistrates Court to aggravated assault of an officer.
The court was told Jessen punched the officer at the rear of the Dublin Hotel at Glenelg last May after police were called to break up a fight.
Prosecutors said it was an aggressive attack on someone performing their duty.
The court was told Jessen was a former New Boyz street gang member and was convicted of affray over a fight between rival gangs in Hindley Street in the city.
He will be sentenced next month.
Jessen recently visited Focarelli in the Royal Adelaide Hospital and, outside court today, Jessen said the pair remained good friends.