What unfolded next almost cost Petrellis his life inside his Upper Gulph Road, Upper Merion, home. Luckily, he came away only with stitches.
By his account, Petrellis opened the door for a man who was asking for someone named "Mark".
"I said there's no Mark here. Before I knew it, he pulled out a gun and pushed me in."
Petrellis said he grabbed the man to control the gun and the two began to struggle violently.
"He pistol whipped me," Petrellis said, "there's blood all over the marble. We were fighting."
Then, Petrellis said a second man came in and he continued to struggle against both, when one of the men put him in a headlock.
"I was was scared," Petrellis recalled. "I was fighting back with them. Although I'm 83-years-old, I'm very feisty. They didn't expect that."
Petrellis estimates he fought back for about six minutes, although it, "felt like an hour."
"It seems like a long time when you got a gun at your head. I never experienced anything like this."
Petrellis said the men ripped a ring off his pinky finger, and stole $1,500 then eventually fled.
"They said, 'We better get the (expletive) out of here,'" according to Petrellis.
Petrellis went to Paoli Hospital where he was stitched to close the pistol-whip wound above his eye. He said his neck is still sore.
When asked why he decided to fight, Petrellis said, "I could tell they were punks, that they didn't know what the hell they were doing."
He said the men came to the house, which is near a school, about 2:30 p.m. - just as buses were lining up, halting their escape for a bit.
They fled in a car with the license plates removed. He believes he was "setup" by someone who knew him, or who knew of him. He doesn't believe more experienced thieves would have come to house that time of day.
"I'll never forget their faces," Petrellis said.
He describes one as a smaller Hispanic-looking male. The second he describes, a "big guy, looked like an Irishman, a big brute."
Petrellis said he wanted to tell his story to warn other elderly residents to never answer the door to strangers.
Police ask anyone with information to call 610-265-3232.
Contact staff writer Mari A. Schaefer at 610-892-9149, email@example.com or @MariSchaefer on Twitter.