An armed gang picked the wrong victim when they tried to steal a £100,000 Range Rover belonging to the boxer Amir Khan.
The former world champion and his brother Haroon, who is also a boxer, were set upon in the early hours of yesterday as they returned to the vehicle following a night out.
A gang of at least six men used a 4x4 car to try and block the brothers' path but the Khans reportedly fought back and their assailants eventually fled from the scene in Birmingham.
A witness to the incident told The Sun newspaper: "One (of the men) slapped Amir and clipped him on the lip but Amir pulled back and knocked him out cold.
"Then five of them went for Amir and Haroon but they stood their ground and these guys got dropped one by one."
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All the Khan brothers were doing was defending their property, the witness said.
They had travelled to Birmingham from Bolton, their home town, for a gig.
Police confirmed they had been called out following reports of a fight but said nobody had come forward.
Amir Khan has his sights set on winning back his world light-welterweight title but has also promised to guide his younger brother to a world title.
He told Telegraph Sport last week: "I think Haroon will be a world champion - we'll definitely keep it in the family."
Haroon's professional debut will be in Manchester in January.