The so-called Rim Fire, which is burning mainly in the Stanislaus National Forest west of Yosemite, nearly doubled its footprint in the park yesterday and later crept closer to thousands of homes.
After advancing on a reservoir for several days, the flames reached the shores of a reservoir but the San Francisco Public Utilities Commission said in a statement there was "little risk for direct impacts" on the city's water supply because of the rocky terrain.
Meanwhile, a firefighting force of some 4100 personnel, backed by bulldozers and water-dropping helicopters, continued to make headway in their drive to encircle and suppress the flames.
Containment lines had been established around 20% of the fire's perimeter, though the overall area of the blaze continued to grow as much of the firefighting effort focused on structure protection.
The Rim Fire has charred nearly 184,500 acres - an area larger than the land mass of Chicago - since it erupted on August 17.
The blaze had scorched more than 40,000 acres of Yosemite, forcing the closure of some campgrounds in the more remote northern part of the park and the main entrance road from the direction of the San Francisco Bay area.