The Steelers didn't even cross midfield until a last-gasp drive to end the half ended with a 49-yard Shaun Suisham field goal.
Any boost the kick provided Pittsburgh disappeared during the first 10 minutes of the third quarter.
San Diego converted five straight third downs during a clock-chewing drive, including a 17-yard burst up the middle by Ronnie Brown on third-and-13 from the Pittsburgh 29.
The play wasn't designed to get a first down, but a pair of missed tackles helped the Chargers extend the drive. Three plays later Rivers found Michael Floyd for a 3-yard touchdown to make it 20-3.
The 17-play march ate up nearly 10 minutes. The Chargers needed barely 10 seconds to put the game away.
Backed up yet again after Floyd's score, Roethlisberger tried to hit Brown on a screen pass. The ball smacked into tight end David Paulson's rear and rolled into the end zone, where Quentin Jammer fell on it.
The play appeared to be overturned on review. Pittsburgh's offense ran back onto the field when referee Scott Green popped out from under the hood.
Instead, Green pulled a switcheroo, confirming the score to give the Chargers a 27-3 lead and a brief moment of joy in a year full of chaos.
"Yeah we've been struggling the last five weeks," tight end Antonio Gates said. "But at the end of the day it only came down to one or two plays in those games we lost. Today, we just were playing the way we were capable of playing, the way we know how to play."
Meanwhile the Steelers will have to find a way to move forward after one of the most demoralizing defeats of Tomlin's six-year tenure. Pittsburgh still controls its playoff hopes after Cincinnati lost to Dallas.
If the Steelers can win their final three games, they'll play into January. That's too far down the road to think about. At the moment, they'd just like to get back to playing something resembling competitive football.
"''Those of us that have been around here, we've got to find a way to pick it up and finish strong," defensive end Brett Keisel said. "We've got to bounce back. We've got to respond from this performance."
NOTES: Steelers LG Willie Colon left the game with a left knee injury. Maurkice Pouncey moved from center to guard, with Doug Legursky replacing Pouncey at center ... Roethlisberger became the 25th quarterback in NFL history to surpass 29,000 passing yards and 1,000 career rushing yards ...San Diego WR Michael Spurlock had seven receptions for 64 yards, with five of the receptions turning third downs into first downs ... Steelers WR Plaxico Burress had an 18-yard reception in the first half, his first reception since being re-signed by the Steelers two weeks ago ... San Diego held the ball for 36:45, the most by a Pittsburgh opponent this season.
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