The offence happened after Dale Brown, 41, was confronted by a known foe near Burger King in Market Square.
A court heard the man repeatedly put his face close to Brown’s, who finally retaliated by getting to his feet, grabbing him - and punching him once.
Prosecutor John Graham admitted Brown, of Swan Lodge, High Street East, Sunderland, had not sparked the confrontation at 3pm on Saturday, March 20.
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But he said there was no excuse for his aggression – captured on CCTV - that followed soon after at the rear of the former Joplings store.
Mr Graham added: “To be fair to the defendant, for quite a few minutes he is not the aggressor.
“The incident goes on for some minutes. There’s quite a lot of footfall at the time, there are children and parents passing by.
“They end up in Thomas Street, and this where the crown would say Mr Brown was culpable.
“There is pushing and shoving and some rolling around on the floor.
“In the footage, Mr Brown delivers two punches that knock the other man down.
“He then kicks him when he is prone. The man lies still for some seconds.”
Mr Graham said Brown has 23 previous convictions, including serving an 18-month prison term in 2018 for affray.
The court also heard Brown was subject to a community protection notice in relation to offences of criminal damage and shop theft.
Annalisa Moscardini, defending, said: “That’s effectively what happens.
“Mr Brown was under provocation at first, but then he shouldn’t have been having a fight in the area that he was having a fight.
“A lot of it is alcohol related, he is an alcoholic.”
Ms Moscardini added Brown was receiving support for his personal issues and was responding positively.
Brown pleaded guilty to a charge of threatening behaviour and failing to comply with a community protection notice.
Magistrates adjourned the case for reports and he will be sentenced at the same court on Tuesday, June 8.
He was granted bail on condition he resides at Swan House and does not enter Sunderland city centre, other than to see his solicitor.