Sgt. Charlie Spruill said the woman, who lives in the 1600 block of Alamo Drive in South Vacaville, fell asleep with the sliding patio door open and woke up around 7 a.m. to find the screen door partly ajar.
She then encountered a man in her hallway who approached her and sexually assaulted her, Spruill said.
When the suspect told the woman he was going to the kitchen to get a knife, the woman ran out the front door and the suspect fled through the rear door, Spruill said.
A witness last saw the suspect running through Southwood Park, Spruill said. Police searched the area but did not find him. The suspect is described as possibly Hispanic, in his early 20s and approximately 5 feet 8 inches tall with a thin build, police said.
He had a tattoo with an unknown design in dark ink on his left forearm and was wearing a red baseball cap and a red or orange shirt with wide blue horizontal stripes.
Police are reminding residents to lock their doors and windows and to leave their homes immediately and call police if they believe a stranger has entered the home.
Anyone with information about the sexual assault is asked to call detective Dave Marsh at (707) 469-6616.