Crews were concerned about strong Santa Ana Winds and triple-digit temperatures, but the severity of the fire was diminished compared to Wednesday, when the fire began.
The blaze is located near the North Etiwanda Preserve. U.S. Forest Service officials say crews building containment lines around the blaze are focusing on the fire's west edge, where structure protection is in place.
The fire started in Day Canyon north of Etiwanda just after 8 a.m. Wednesday during conditions of low humidity, high temperatures and strong winds gusting up to 100 mph.
When flames moved close to foothill neighborhoods, residents were told to evacuate their homes. Some people stayed behind to soak their property with garden hoses.
Winds prevented the use of water-dropping helicopters or air tankers, and officials say they have not been able to survey the area from the air to get a more precise figure on the number of acres burned.
Two firefighters sustained minor injuries, according to Cal Fire.
A mandatory evacuation order was lifted, but voluntary evacuations remained in place north of Hillside Road from Haven Avenue to Milliken Avenue.
An evacuation center for people and animals was set up at Central Park on Baseline Road and Milliken Avenue in Rancho Cucamonga. Small animals can be taken to Animal Center at 11780 Arrow Route in Rancho Cucamonga.
Road closures include Etiwanda Avenue between Banyan Street and the north city limit and Day Creek Boulevard between Banyan Street and the Northern C.
At least 580 personnel from the U.S. Forest Service, CAL FIRE, Ranch Cucamonga Fire Protection District and San Bernardino County Sheriff's Department are assisting in the firefighting efforts.
The cause of the fire was under investigation.
Chaffey Joint Union High School District schools were scheduled to be open on Friday.
The North Etiwanda Preserve was closed due to the fire.
The Associated Press contributed to this report.