Saturday's AFC Wild Card clash rung familiar in this longstanding rivalry, with defense and opportunistic play deciding which team survived the slugfest.
The Ravens, who needed help in Week 17 to even reach the postseason, provided most of the punishing blows, as they sacked Ben Roethlisberger five times and forced two interceptions in the fourth quarter.
Flacco threw a pair of touchdown passes, including one to Crockett Gillmore that gave the Ravens a 30-15 lead with under eight minutes remaining.
A holding penalty negated a Steelers touchdown on the ensuing drive, and Roethlisberger was picked off in the end zone to seal the outcome.
Harbaugh and Flacco have teamed together for seven playoff victories on the road, and they will need another against the top-seeded Patriots in next week's Divisional Round on Saturday.
The Steelers had won four straight home playoff games but had no running attack with All-Pro Le'Veon Bell sidelined with a knee injury.
Ben Tate, signed just this week, and rookie Josh Harris combined for 44 yards on the ground in place of Bell, who hyperextended his right knee in last week's AFC North-clinching win over the Bengals.