The trial of a man accused of murdering Karen Ashcroft in Maryborough started in Ballarat on Thursday.
Percydale man Ben Wardlaw is standing trial in the Supreme Court at Ballarat over Ms Ashcroft's death in May, 2018.
Witness Kerryn McCallum told the court she saw Wardlaw with an orange knife before the incident.
"She (Ms Ashcroft) got up and that's when he struck her," Ms McCallum said.
"When she got up he said something to her and that's when he struck her."
The witness said "it was really quick" and "I knew he had the knife but it was so quick".
"Karen was walking around ... it didn't look like she had been stabbed until she dropped to the floor," Ms McCallum said.
The jury was told an ambulance took half-an-hour to arrive, while two other people tried to help and gave Ms Ashcroft CPR.
A 13-person jury was empanelled on Thursday. The trial will continue on Friday in front of Justice Rita Zammit.
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