Fire crews began thinning near the Willow Canyon neighborhood and Catalina Highway on Monday, fire spokeswoman Sarah Davis said. The homes, which are about six miles southeast of Summerhaven, were not immediately at risk, she said.
Forest Service officials estimate there is a 75 percent chance the cleared area along the highway will stop the fire.
About 100 full-time residents were evacuated Sunday as high winds and fuel-rich vegetation hampered firefighters battling the blaze. The rest of the home and cabin owners in the area are part-time residents who either left earlier in the week or weren't there during the fire, said Dean Barnella, chief of the Mount Lemmon Fire Department.
The fire hadn't damaged any structures but it had the potential to threaten all 700 homes and 15 businesses on Mount Lemmon, authorities said. If the fire burns up Mount Bigalow, an observatory and radio and television towers also could be threatened.
"We're working hard and doing as much as we can, but it's pretty obvious to us that Mother Nature is in control and we're not," Forest Service spokesman Jim Paxon told the Arizona Daily Star.
The fire was 40 percent contained Monday night. There was no estimate for full containment.
About 913 people, including 20 hotshot crews, were working the blaze, spokeswoman Sharon Brown said.
Two-thirds of the available resources in the Southwest are being used to battle the fire, Brown said.
The estimated cost of the fire was $4,172,831, Brown said.
The Pima County Board of Supervisors declared a state of emergency Sunday which allows the county to request and receive aid for the affected area from state and federal sources, said Supervisor Ray Carroll, whose district includes the mountain area.
Gov. Jane Hull is submitting the declaration to Federal Emergency Management Agency officials in Washington.
Carroll said seeing so much of one of Tucsonans' favorite getaways on fire was gut wrenching.
"It's like seeing a friend dying of cancer," he said.
The so-called Bullock fire, was first spotted Tuesday in the Redington Pass area and is believed to be human-caused.
Due to the extreme fire danger, nearly all access to 1.8 million acres of the Coronado National Forest in southeast Arizona and southwest New Mexico has been closed.