The top-seeded Golden Hawks advance to host No. 5 seed Garnet Valley in their second consecutive trip to the semifinals.
"We're following our leaders from last year," running back Ryan Whiteley said. "But we're taking it one game at a time and hoping to have a good season."
The first seven minutes of the third quarter were similar to the previous two: big plays and big mistakes by both teams.
C.R. South quarterback Brian Donnelly escaped for a 43-yard touchdown with 6 minutes, 49 seconds left in the quarter, putting his team ahead by 21-13.
Four plays later, on third and 20, Downingtown East quarterback Kyle Lauletta hit Todd Dais over the middle for a 75-yard touchdown pass.
It was Downingtown East's first and only big play of the second half.
Donnelly scored again, on a 1-yard run up the middle, with 11:23 left in the game.
On the ensuing drive, Andrew Cohen and Steinmetz recorded sacks as Downingtown East, the No. 9 seed, went three and out.
On the Cougars' next possession, and final chance to come back, they were again stonewalled by the C.R. South defense. And Steinmetz once again came up with a big sack to stop the Cougars.
"Once we were able to settle in and see what they were doing, it allowed our front four to be more aggressive," South coach Vince Bedesem said. "They have the weapons, so it was important for us to nip it in the bud.
"We just had to let the kids play."
The defense also came up with a big stop on fourth and goal from the 2-yard line late in the second quarter.
The team pointed to that drive as a momentum shifter after allowing the Cougars to score on an 82-yard kickoff return, by Jay Harris, in the first quarter and losing an onside kick on the ensuing kickoff.
"That fourth-down stop at the end of the first half was big," Steinmetz said. "We definitely carried that into the second half."
Downingtown East 7 6 6 0 – 19
Council Rock South 14 0 7 10 – 31
CRS: Whiteley 2 run (Solis kick)
DE: J. Harris 82 kickoff return (White kick)
CRS: Alimenti 29 run (Solis kick)
DE: Angeline 2 run (kick block)
CRS: Donnelly 43 run (Solis kick)
DE: Dais 75 pass from Lauletta (run failed)
CRS: Donnelly 1 run (Solis kick)
CRS: FG Solis 32
Contact Chris Melchiorre at rallysports@phillynews.com.