A man fractured his skull after being knocked to the ground during an argument in the street.
The 43-year-old was punched twice in the face while rowing in Borage Close, Crawley, at about 10pm on Saturday.
He was taken to the Royal Sussex County Hospital in Brighton and has been detained for treatment.
His injury is not considered life-threatening and he is stable.
His assailant is described as a man in his early 20s, 5'10", skinny but muscular build, with cropped blond or light brown hair. He was wearing dark coloured tracksuit bottoms.
Anyone who saw what happened, who can name the assailant or who may have other information, is asked to email 101@sussex.pnn.police.uk or phone 101, quoting serial 1643 of June 27.
Alternatively they can contact Crimestoppers on 0800 555 111.
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