The Baltimore Ravens' defense has been an immovable force in the second half of games this season.
When the offense is scoring points, this could pave the way for a deep postseason run.
Baltimore (3-1) kept the Steelers reeling and is tied with Cincinnati for first place in the AFC North.
The Ravens' revamped passing attack kept Pittsburgh on its heels for most of the game.
"We don't even have all of our tight ends healthy, but we have a lot of versatility at that position, we have a lot of versatility at our wide receiver group," Flacco said. "I think our running backs do a great job of rotating in there, and they all do their things that they do really well. It is exciting that we have that. And our job is to go out there and continue to execute the way we've been executing."
The Steelers had won 12 straight in primetime games. This time, they were clearly outplayed by Baltimore.
The Ravens got off to a quick start, opening a 14-0 lead on a 33-yard touchdown pass from Flacco to John Brown and a 3-yard run by Alex Collins.
Brown finished with three catches for 116 yards.
The Steelers' secondary has been susceptible to big plays this season. Baltimore managed 451 yards of total offense, compared to 284 for Pittsburgh.
"Every game is different, but we aren't winning the game. Simple as that," Steelers defensive end Cam Heyward said.
The Steelers managed to get back in the game, courtesy of a Collins fumble at the 1-yard line. That swayed the momentum back in their favor and the Steelers tied the game on a pair of field goals by Chris Boswell and a 26-yard pass from Ben Roethlisberger to Antonio Brown, followed by a 2-point conversion.
However, those were the only points Pittsburgh could manage against an impenetrable Ravens defense.
"We went a lot of no huddle and got into a rhythm and when you're converting and getting first downs you can get new sets of downs and more things happen," Roethlisberger said. "We did not make enough plays in the second half to do that."
The Ravens pulled away with four second-half field goals by Justin Tucker. Baltimore kept the ball on the ground most of the way to control the clock.
Despite the early success, Ravens head coach John Harbaugh has tempered his enthusiasm.
"I think it's one game at a time, just like it's one play at a time and thinking about it, it's great for the fans," he said. "The fans have a lot of fun with it. They enjoy that and I hope they feel that way, have a lot of confidence in us."
Meanwhile, the Steelers are dealing with numerous off-the-field issues. Running back Le'Veon Bell is still holding out and Brown had a confrontation with an assistant coach.
However, Steelers head coach Mike Tomlin is not looking for excuses.
"I am just trying to win the games as we sit here today," he said. "We have some things we need to improve on and we're continually working to do so."