It happened around 9 a.m. at the Girard Court Apartments on the 2100 block of North College Avenue.
Police say there was some type of altercation after a man and woman answered the door.
"At some point during that contact, a physical struggle ensued, at which time the writ serve discharged his weapon, striking the female resident one time in the head," stated Lieutenant Jason Hendershot with the Philadelphia police.
"It was like, 'Boom boom!'" said neighbor Jennifer Rivera, who says she knows the victim. "I saw her coming out and she was holding her head with a cloth, and she was alert because I saw her eyes open."
According to officials, the woman may have been armed with a weapon, possibly a knife.
"Allegedly, there was a knife involved and that's why he discharged his firearm, so we have to figure that out," said Hendershot. "There is some video in the apartment complex that we will be reviewing. There's also some body camera footage of the writ server that we have, that we have a search warrant for."
After shots were fired, officials say the woman's husband barricaded the two of them inside the apartment. Police then ordered the man to open the door and let his wife out for medical treatment.
"She didn't deserve that, no one does," said Rivera. "Why he shoot her? Why he shoot her?"
Police say the 35-year-old woman is in critical condition.
Her husband was not hurt during this incident.