About 230 crew members - 40 of them involved in aerial operations - were fighting the fire in the eastern part of sparsely populated upper Michigan, according to the state Department of Natural Resources.
Agency spokesman Dean Wilson said the fire, north of Newberry, was about 47 percent contained.
"They're holding the line pretty well," said Wilson, who also noted that, as of 6 p.m. Sunday, there were no reports of injuries.
Lightning delayed air operations until about noon Sunday. But the addition of four Michigan National Guard helicopters to the fleet of two air tankers and a helicopter was making a difference, particularly in trying to shield structures from the flames.
Dozens of buildings have been destroyed, and efforts to assess the damage are still ongoing. Wilson said a partial count showed the fire had destroyed 28 homes, 19 garages and 14 outbuildings.
On Sunday morning, the state estimated that the fire had covered 20,255 acres, slightly less than Saturday's estimate of about 21,000 acres. There was no update Sunday evening.
The Duck Lake Fire started with a lightning strike Wednesday. The blaze is in a stretch of forest that touches Lake Superior, about 75 miles northeast of the Mackinac Bridge. The area is remote and near inaccessible in places.
Fewer than 100 people have been reported evacuated, and many of the area's homes are seasonal and were unoccupied at the time, Wilson said.
"Today's fire suppression activities will focus on holding existing secure line, fortifying established line put in yesterday, and extending fire lines north from the fire heel along both the east and west flanks," the DNR said in a statement Sunday.
The south end of the fire was 14 miles north of Newberry and seven miles west of the Tahquamenon Falls State Park campgrounds, the DNR said.
Mop-up operations have been under way in the Seney National Wildlife Refuge in neighboring Schoolcraft County, where a fired burned about 3,400 acres, or about 5.3 square miles. That blaze was reported Monday and also blamed on a lightning strike.
Gov. Rick Snyder declared a state of disaster in Luce and Schoolcraft counties. The declaration permitted authorities to order evacuations, establish a fireworks ban in the two affected counties and ban outdoor burning in 49 counties.
