Anthony Tomlinson said Alison Wilson decided to act because a baby was being "swung round" as the child's mother rowed with Stephen Duggan in Widnes at about 23:20 GMT on 7 March.
He told Liverpool Crown Court seeing that so late at night was "strange".
Duggan, 28, of Runcorn, denies murder.
Mr Tomlinson, who was travelling in a taxi with Ms Wilson when the pair saw the argument, said Duggan and the woman were "tussling" over the baby's car seat.
"One was trying to take the car seat off the other. It was not gentle pushing," he said.
The jury heard he paid the taxi driver as Ms Wilson intervened and "did not feel threatened at all" as he got to the scene, where the mother-of-two was pushing Duggan away from the woman.
He said that without warning, he was then hit over the head.
"I think I was knocked unconscious straightaway. I remember I was lying on the floor and I came round. My first thought was for Al."
Visibly upset, he said the 36-year-old was "standing with her hand on her neck", adding that she had "a look of shock on her face".
"I told her to keep the pressure on it. I didn't think it was bad it as it was."
Ms Wilson was taken to Whiston Hospital, but died there six days later. Mr Tomlinson required plastic surgery on his head wound.
The court also heard from the woman who Duggan was arguing with, who cannot be named for legal reasons.
She said they had become involved in a "tug-of-war" over the car seat after the pair had left a gathering at his father's house.
She told the court the pair and Ms Wilson all had hold of the car seat at one point, before the baby fell out and Duggan punched the woman in the face.
Asked what then happened to Ms Wilson, the woman said she "just saw her hit the floor", but added that she did not see if Duggan was holding anything.
The 28-year-old has admitted assaulting the woman, but denies murdering Ms Wilson, wounding Mr Tomlinson and causing actual bodily harm to the baby.
The trial continues.
A woman who was killed after she stepped in during a street row between her alleged murderer and woman with a baby intervened because "it didn't seem right", her friend told a court.