The victim, in his 40s, was punched in the head in Twyford Avenue.
He was assaulted by another man between 11.53pm and 11.59pm on Saturday night.
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It happened near a newsagents, and then again at the junction with Meyrick Road, where he was knocked out.
He was rushed to hospital with head injuries.
Hampshire police has described the attacker as being white, in his mid 30s, 5ft 8ins tall, medium build, and with blonde/mousey brown hair.
He was wearing a white t-shirt and jeans on the night of the assault, and was previously stood in a group of between four and five people before the attack, they added.
DC Jennivy Morris said: ‘This was an unprovoked attack on a busy road which left a man with facial injuries that needed hospital treatment.
‘We believe there were around 15 people in the vicinity when the assault happened.
‘If you know who did this, or have any information which could aid our investigation, please get in touch.
‘Likewise, if you filmed the incident or have CCTV, dash cam or ring doorbell footage showing what happened we would like to hear from you.’
Anyone with information is advised to contact Hampshire Constabulary, quoting 44220307623.
You can also submit a report online via the Hampshire police website.
Anonymous information can also be sent to Crimestoppers anonymously through their website, or through their phone number on 0800 555 111.