Thurman, who said he moved in with his grandparents in the Overbrook section, was 4 for 13 from the field before his game-winner from near the foul line, including 3 for 9 from beyond the arc.
"My three-point shot wasn't on, so I was looking for a mid-range shot," the 6-foot, 185-pound senior said.
In a Friday quarterfinal, at a neutral site and a time to be determined, the Chargers (18-11) will play Archbishop Carroll (23-4). The Patriots advanced Tuesday with a 59-55 victory over District 2 champ Abington Heights.
Against Holy Ghost, Thurman, who added two assists and two steals, was supported by junior guard Devante Truitt (14 points, five steals) and 6-6 senior forward James Suber (12 rebounds, nine points, four blocks).
"I think we did a very good job of slowing the game down," said Lewis, whose squad committed just two turnovers.
After Thurman's go-ahead bucket in OT, the Firebirds, unable to get off a shot and trapped on the left wing, called a timeout with just 2.3 seconds to go. Matt England's three-point try at the buzzer, with Thurman defending, fell way short.
Down by 43-37 in the fourth quarter, Holy Ghost (23-7) rallied to force the extra session. Zahir Watkins evened things on a right-side drive with 0:21 to go.
"It was anybody's game down the stretch, and they just made one or two more plays than us," Firebirds coach Tony Chapman said.
England, who sat out the last seven minutes of the first half after picking up his third foul, posted 10 of his 20 points in the fourth period.
Watkins and reserve Joe Braun (16 boards, three blocks) each scored nine points.
Holy Ghost Prep 12 7 9 16 8 – 52
Phila. Electrical 17 8 8 11 9 – 53
HGP: Mike Kirby 7, Matt England 20, Zahir Watkins 11, Joe Braun 11, Liam Brennan 3.
PET: Larenz Thurman 18, Jihad Barnes 6, James Suber 9, Nysir Marshburn 4, Devante Truitt 14, Anthony Harris 2.