Garry Delaney, 35, who denied murder, lifted Paul Price off his feet with the blow, the Old Bailey was told.
The trial heard Mr Price, 23, had gone to remonstrate with Delaney who had thrown his friend James Farrell out of a hotel in Woodford, east London.
Delaney knew the punch would cause "really serious harm", the court heard.
"Delaney was seen to hit him in the face with an uppercut which lifted him off the ground"
Mr Price fell backwards and hit his head on the ground, fracturing his skull.
The trial heard he had been turned away with Mr Farrell from an over-35s disco in October 2005, for not meeting the dress code.
They went to the hotel's bar instead, but the barman thought Mr Farrell was being "troublesome" and asked Delaney to remove him.
The trial heard Delaney then appeared to spray Mr Farrell in the face with a substance that made him scream.
He dragged him outside and dumped him in shrubbery which led Mr Price to go outside to "remonstrate verbally" with Delaney, the court was told.
"Delaney was seen to hit him in the face with an uppercut which lifted him off the ground," said Alan Suckling QC, prosecuting.
He said Delaney was "an experienced professional boxer who would know such a blow done deliberately by him was likely to cause really serious harm".
Delaney, from Newham, east London, had fought at light heavyweight and cruiser weight and was working as a licensed doorman when he killed Mr Price.
He is due to be sentenced on Friday.
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