Mancuso, who had served time for murder, pulled a knife. Testa, 77, a retired hospital security guard - and a well-liked man, his neighbors said - stepped between the men.
But he quickly stumbled back inside and collapsed to the floor.
"Ro, he stabbed me three times. Hurry up and get a towel," he told his sister Rose Knowles, 80, whom he lived with.
Testa died at the Hospital of the University of Pennsylvania at 49 minutes past midnight, one of six people killed in the city between Saturday afternoon and Sunday morning, police said. Others were wounded in a night of violence that had veteran detectives shaking their heads.
Saturday's three killings brought the 2011 homicide toll to 320, the most in three years.
The holiday killing began around 4 p.m. Saturday, when Kevin Moore, 26, was shot six to nine times in Hunting Park while sitting in the front seat of his silver Jaguar. Police had made no arrests as of Sunday night.
Just before midnight, police spotted a green Hyundai idling with its lights on in the 5600 block of North Second Street in the Olney section. Arlette Aguero, 31, of the 4700 block of Tampa Street, and Alejandro Garro, 34, of the 7300 block of Dungan Road, were found shot to death in the front seat.
At 1:30 a.m., Gerard Market, 48, was found fatally shot on the 4100 block of Orchard Street, where he lived.
Around 3 a.m., a 23-year-old man whose identity had not been released Sunday night was found shot in a shoulder at Eighth and Callowhill Streets. The bullet traveled downward, and he died within the hour.
And police ping-ponged across the city throughout the night, responding to nonfatal shootings.
On the 5200 block of Sansom Street, a man opened fire on a New Year's Eve house party, angry over the $5 cover charge. He wounded three people in their arms and legs.
In Testa's killing, Mancuso, of the 1000 block of Ritner Street, was arrested Sunday morning on Jessup Street and charged with murder.
Mancuso has a criminal record for weapons offenses, assault, and burglary. In 1993, he was sentenced to six to 12 years in prison for third-degree murder in the death of Patrice Bowens, 28, of Asbury Park, N.J. Bowens was working as a prostitute in 1990 when Mancuso shot her once in the head, police said.
She was found dead in a South Philadelphia alley.
It was unclear from court records Sunday how long Mancuso served for that killing.
Mancuso often hung around with his brother-in-law Michael Guagenti, who lives two doors down from Testa. Neighbors said they had clashed with Guagenti since he moved there. He angered the block by trying to reserve spaces for his four cars, they said. Guagenti and Mancuso were often combative, neighbors said.
On Sunday morning, Testa's sister Rosemary sat weeping on her couch, surrounded by family, her brother's blood caked on the wooden floor.
Testa had been married once and had three children, his sister said. For years, he worked as a technician for Sealtest Ice Cream, and then as a shoe salesman. He retired as a security guard about seven years ago, she said.
He spent recent years caring for his house-bound sister. "He did everything for me," she said. "He cooked and cleaned and gave me my medicines. He was up and down those stairs on his bad legs every night to check on me. . . . What a man he was."
He had decorated the house with a small Christmas tree and white wreath. He was going to tape the Mummers Parade so they could watch it together.
Now, his sister, cried in the arms of a relative, "We don't have him anymore."
In the afternoons while his sister rested, Testa would visit friends at a barbershop at Sartain and Oregon.
He would buy his lottery tickets, always playing 110, his birthday numbers - he would have turned 78 on Jan. 10 - and then talking sports and joking about losing weight while he ate a big lunch.
Testa's shocked friends sat outside the shop on Sunday. He was a good and decent man, they said. And they worried about Rose.
Santo Procopio, owner of the shop, said, "I don't know what she'll do without him."
Contact staff writer Mike Newall at 215-854-2759 or email@example.com.