Girls Aloud singer Cheryl Tweedy was punched twice in the face by a nightclub toilet attendant before she retaliated, one of her fellow band members told a court.
Miss Roberts denies she made up her story to help Miss Tweedy
Nicola Roberts, 18, told Kingston Crown Court that Sophie Amogbokpa threw the first punch in the row and then hit Miss Tweedy again before her friend hit her back.
Miss Roberts said Mrs Amogbokpa first punched her fellow band member as she rummaged around in her handbag to get some change to pay for a handful of lollipops she had taken from a display in the ladies lavatory.
"She (Miss Tweedy) just retaliated in self defence as anybody would if they had been punched in the face," Miss Roberts told the court.
Miss Tweedy, 20, from Heaton in Newcastle, has denied assaulting Mrs Amogbokpa in the incident in the Drink nightclub in Guildford on 11 January.
Prosecuting Patricia Lees suggested to Miss Roberts that she had invented the story in the nightclub as a "lifeline" for Miss Tweedy because she knew her friend was in "real trouble".
But Miss Roberts said: "At the end of the day it's my life, my career, I'm not going to lie for anybody."
She went on to tell the court she had not heard Miss Tweedy use any racist abuse during the incident.
She admitted her friend was drunk but denied she had been "unsteady on her feet".
Miss Tweedy won her place in Girls Aloud in a television show
The court also heard from 26-year-old Lucy Fairbrass who said she was a regular visitor to the Drink and has previously had a "confrontation" with a toilet attendant there.
She described the woman involved as being black with short black hair and said she had said repeatedly "do you think I'm stupid?" after Miss Fairbrass sampled some perfume from a display in the toilets.
The trial heard from two people who had worked with Miss Tweedy before she became famous .
Anoop Bedi, for whom she worked in a cafe in Newcastle, and songwriter Ricky Hanley, both told the court Miss Tweedy was not a racist.
Miss Tweedy denies racially aggravated assault occasioning actual bodily harm and an alternative charge of assault occasioning actual bodily harm.
The trial was adjourned until Friday.