The man accused of murdering banker Oliver Dearlove had "bad intentions" when he delivered a single powerful knock-out punch, a court heard.
Trevor Timon, 31, allegedly became "very angry" and was "looking for a fight" before landing the blow which felled the 30-year-old victim in the early hours of August 28 last year.
The court heard how Mr Dearlove had spent the evening drinking beer, watching football and sharing a curry in Blackheath, south east London, with old Portsmouth University friends.
As they walked along Tranquil Vale to catch cabs home, they had a "friendly chat" with a group of women known to the defendant who had just left Morden's nightclub.
Following the brief exchange, lasting just seconds, Timon came up to Mr Dearlove and his two friends and demanded to know what they had been talking about, jurors have heard.
Giving evidence, the victim's friend Andrew Cook said: "There was this guy.
"I don't know where he came from. He was just upon us all of a sudden. He came out of nowhere.
"He just started getting up in our faces. He started getting aggressive, shouting at us something like 'Who the f*** are you?'
"He was very angry and agitated. He pushed me. He got really close to us pointing and shouting at us, being really abusive.
"He was looking for a fight. Not a nice guy. Trying to start a fight. We didn't know why, it all happened so quickly."
Mr Cook, from Clapham, south London, said he held his hands up as if to say "what's going on here" just before his friend was punched in the face.
Describing the attack, he said: "I believe with his left hand he hit him as hard as he could. He really put his whole body into it. It was very forceful, very quick, very powerful.
"He had bad intentions, basically. It was more of an aggressive hook. He put everything he could into it. Then Oli just fell to the ground."
He told jurors Mr Dearlove made a snorting sound and then lay unconscious in the street not breathing.
Someone called an ambulance and passed the phone across to Mr Cook who took instructions on resuscitation before handing over to a passing first aider, jurors heard.
Under cross examination, Courtenay Griffiths QC, suggested someone had commented "she's with the half chap", in reference to the mixed race defendant.
But Mr Cook denied it, saying: "He just came out of nowhere. I did not know he was with the girls."
The court has heard Mr Dearlove, who lived with his girlfriend in New Eltham, died within 24 hours of the attack.
The court has heard he spoke to one of the women about her sore feet as she walked up the hill holding her new high heels.
When the woman told him it was her first night out since giving birth, he commented she looked "well" and admired her baby pictures, before Timon stormed in, jurors have heard.
Timon, from Plumstead, south east London, has admitted manslaughter but denies murder.