Alex Smith threw a 1-yard scoring pass to Vernon Davis and Frank Gore ran for a 5-yard touchdown as the San Francisco 49ers pulled away in the second half for a 20-3 victory over the visiting Pittsburgh Steelers on Monday night.

San Francisco (11-3) got a stellar defensive effort, forcing four turnovers and sacking Steelers quarterback Ben Roethlisberger three times in snapping Pittsburgh's four-game winning streak.

San Francisco extended its streak of not allowing a 100-yard rusher to 36 games and became the only team in history to not surrender a rushing TD in the first 14 games of a season.

The loss was a costly one for the Steelers (10-4), who fell a game behind New England for the top overall seed in the AFC and dropped into a tie atop the AFC North with Baltimore. The Ravens hold the tiebreaker by virtue of a season sweep.

The 49ers intercepted Roethlisberger three times and used a dominating game of field position to stifle the Steelers, who did not start beyond their 20-yard line on nine consecutive drives.

Even when Pittsburgh finally got on the board, cutting the lead to 6-3 on Shaun Suisham's season-long 51-yard field goal with 6:08 left in the third quarter, San Francisco responded with the game's first touchdown on the ensuing possession.

Smith was 5 of 5 on the drive, hooking up with Davis for 31 yards and 21 yards before rolling out and hitting his tight end from 1 yard out for a 13-3 lead with 3:44 left in the third.

The 49ers then took advantage of two huge mistakes to break the game open. A sack of Roethlisberger and fumble recovery by Justin Smith set the Niners up at Pittsburgh's 17-yard line.

The Steelers stiffened and forced a short field goal attempt, but Lawrence Timmons was penalized for leaping on the play, giving the 49ers a first down at the 5. Gore plowed in two plays later for a 20-3 lead with 9:06 to play.

Roethlisberger, playing with a high ankle sprain, threw for 330 yards on 25-of-44 passing. Smith was 18 of 31 for 187 yards and a TD.

The start of the game was delayed 20 minutes by a power outage at Candlestick Park, and a second outage delay the contest for 16 minutes early in the second quarter.

Offense was at a premium in the first half with two field goals by David Akers accounting for San Francisco's 6-0 lead.

Akers’ 38-yarder with 12:13 to go in the second quarter gave him 141 points on the season and eclipsed the franchise single-season record set by Hall of Famer Jerry Rice (138) in 1987.