Full containment was expected by Dec. 15, fire officials said.
The quick-moving brush fire consumed thousands of acres and triggered evacuations of homes and schools after it was sparked on Tuesday and exploded in acreage that same day into Wednesday.
The blaze began about 9:30 a.m. Tuesday near the 2500 block of Rye Canyon Loop. Less than two hours later, it had spread to 5,000 acres, according to the Los Angeles County Fire Department.
About 775 firefighters responded to the burn area, which was initially north of Newhall Ranch Road and west of the 5 Freeway. The interstate was temporarily closed in the burn area, but traffic resumed in both directions by 2 p.m.
After being evacuated, residents of the Stevenson Ranch community were allowed to return to their homes.
Simi Valley residents were on alert as the flames marched southwest toward that community. All Simi Valley Unified School District campuses were closed Wednesday "due to poor air quality and potential fire dangers from neighboring events," the superintendent, Dr. Jason Peplinski, said in a statement.
By Friday, the inferno had charred 6,049 acres west of Valencia.
Favorable conditions helped firefighters make progress, according to Cal Fire officials, but they warned that the red flag warning and wind conditions still pose a threat to the area.
One structure was destroyed in the blaze and one firefighter was injured, according to fire officials.
The cause of the fire was still under investigation.