The girl said she and a friend were walking down Santiago Boulevard when a mumbling man turned and attacked.
"He walked across the street and we immediately thought that he was a troubled man," Melissa said.
"We were suspicious about him, so, we were like, 'OK, well, you know, stay calm, don't make eye contact with him," Emily said, a friend who was walking with Melissa when the incident occurred.
"And then I saw him with the knife, like, about to lunge into her," said Emily. "And then, we both screamed at the top of our lungs ... like bloody murder."
Melissa suffered two stab wounds to her rib area, underneath her right arm.
The girls ran to a nearby gas station, and people chased the suspect, identified as 49-year-old David Eddy and held him down until deputies arrived.
Authorities said Eddy is a transient who has spent time in at least two psychiatric facilities and has been in trouble with the law most of his life.
"Assault with a deadly weapon, different assaults ... one including an assault on a peace officer," said Jim Amormino, Orange County Sheriff's Department.
Eddy now faces attempted murder charges.