* Containment slips to 1 pct from 5 pct day before -forest service
* Reinforcement firefighters expected to arrive Friday
* Country's fastest-growing fire reaches edge of Yosemite (Updates to show containment dropped during the day as fire rapidly spread)
By Laila Kearney
SAN FRANCISCO, Aug 22 (Reuters) - Firefighters struggling in rough terrain lost ground against a raging wildfire that on Thursday had reached the western edge of Yosemite National Park in California, one of America's premiere outdoor destinations, the U.S. Forest Service said.
As flames crossed into the park, mandatory evacuation orders were issued for an undisclosed number of people in the burn zone near the Stanislaus National Forest, the Tuolumne County Sheriff's department said.
The blaze, which was burning mostly just outside Yosemite, grew from 54,000 acres (22,000 hectares) to 64,000 acres during the day and was just one percent contained by nightfall, the agency said on its website.
That was a drop in containment from the night before, when the blaze - then smaller by two-thirds - was five percent contained, the Forest Service said.
The so-called Rim Fire, named after a popular Stanislaus National Forest lookout point, Rim of the World, was burning on rugged and remote terrain, which has made it challenging for firefighters to haul in hoses, said U.S. Forest Service spokesman Trevor Augustino.
"The terrain is so difficult that you can't go into direct attack," he said, adding that a lot more firefighters were expected to arrive on Friday as reinforcements to the 1,360 already on the job.
The blaze is the fourth-largest and fastest-growing wildfire in the nation, said National Interagency Fire Center spokeswoman Robyn Broyles. It is one of 50 large wildfires currently burning throughout the U.S. West.
California Governor Jerry Brown on Thursday declared a state of emergency in Tuolumne County, where the fire is burning. The fire has destroyed two homes and seven outbuildings since it broke out on Saturday.
By Thursday, the fire had spread onto the western edge of Yosemite National Park, Augustino said, but he could not immediately provide further details.
Some 19 recreational areas in Stanislaus National Forest, including campgrounds and recreational vehicle campsites, have been evacuated due to the fire.
Separately, residents on a street directly threatened by the fire were ordered to evacuate, according to the U.S. Forest Service website. Details on the number of evacuees were not immediately available.
Residents of about 2,500 homes in the area have received evacuation advisories, which strongly urge residents to leave but do not make evacuation mandatory, said Tuolumne County Sheriff's Sergeant Scott Johnson. Typically, about 75 percent of residents evacuate after receiving such an advisory, he said.
Two evacuation centers were set up for residents, Augustino said.
"There are a lot of little pockets of residences throughout this area," he said, adding that the fire has started to spread to private land.
Yosemite National Park, spanning 750,000 acres (300,000 hectares) attracted nearly 4 million visitors in 2012, according to the park website. It is famous for its valleys, waterfalls and polished domes.
Park officials on Tuesday were forced to stop westbound traffic on Highway 120, one of four access routes to the park, due to the fire. But as of Thursday morning, they had no plans to close the park, said Yosemite park ranger Scott Gediman.
The cause of the Rim Fire is unknown and under investigation, Augustino said. (Editing by Alex Dobuzinskis, Bill Trott and Ken Wills)