Matthew William Lawson, 25, pleaded guilty on Tuesday in Perth Magistrates Court to attempting to unlawfully kill Trae Black, 29.
Police said Lawson and Mr Black were involved in a verbal stoush on Monday at a construction site in the affluent suburb of City Beach.
Mr Black fled the site on foot, with Lawson in pursuit driving a silver Mazda BT-50 utility.
It is understood Lawson struck Mr Black with the vehicle on nearby West Coast Highway, severing his leg, before assaulting him with a hammer.
Superintendent Manus Walsh said Mr Black's life was saved by members of the public, who provided first aid at the scene.
"The victim has sustained significant injuries to his leg region," he said.
"(If he had) not been rendered medical assistance by witnesses at the scene, there is a likelihood that he would have lost his life.
"We are very, very thankful for the off-duty doctor and the member of the public who assisted in helping this victim."
Mr Black was taken to Royal Perth Hospital, where he remains in intensive care in a critical but stable condition.
West Australian Premier Roger Cook has called the attack a disgusting crime. (Richard Wainwright/AAP PHOTOS)
Premier Roger Cook said it was a "heinous, ugly and violent attack".
"This is a disgusting crime," he told reporters on Tuesday.
"I don't think you could ever come across a crime more violent, more physical."
Bearded and wearing a white singlet revealing heavily tattooed arms, Lawson refused to seek legal advice before admitting to the crime during his first court appearance.
"You would appreciate this is an exceptionally serious charge and I suggest it would be prudent to get some legal advice this morning because this matter carries a significant prison term," Magistrate Janelle Scutt said.
"I'm not trying to be condescending, I want to make sure this is a fully informed plea on your behalf."
Asked if he was under the influence of any drugs or if he had any mental health concerns, Lawson said no.
Lawson also refused Ms Scutt's advice to permit her to order pre-sentence and psychiatric reports.
He did not apply for bail and was remanded in custody to appear in Stirling Gardens Magistrates Court on August 21.
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