The pair of pugilists had returned to the car after a night out in Birmingham when six men ambushed them.
The Sun reports that the gang used another 4x4 to block the Khans' path, while another brandished a cosh.
The brothers fought them off and escaped with just cuts and bruises, although the Range Rover's back window was smashed.
An eyewitness told The Sun: "These guys tried to jump Amir and his brother but they picked on the wrong blokes.
"They parked their car in front of the Range Rover. At least one had a cosh. One 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."
Police said they were called out after reports of a fight but have made no arrests.