Alex Smith returned from a five-game absence and completed 17 of 27 passes for 255 yards and three first-half touchdowns on Sunday as the San Francisco 49ers pulled within a game of first place in the NFC West with a 40-21 victory over the visiting Seattle Seahawks.

The 49ers (5-8) came into the day trailing co-division leaders Seattle and St. Louis (6-7) by two games, but capitalized on five turnovers and also took advantage of the Rams' loss to the New Orleans Saints on Sunday to keep their playoff hopes alive.

San Francisco, which lost at Seattle 31-6 in the season opener, scored on six of seven possessions to take a 30-7 lead at halftime. The Seahawks (6-7) turned the ball over three times in the first two quarters, leading to a touchdown and a pair of field goals for the 49ers.

Seattle's Matt Hasselbeck completed 27 of 42 passes for 285 yards and two touchdowns, but was intercepted four times and also lost a fumble. Dashon Goldson's 39-yard interception return for a touchdown gave the 49ers a commanding 37-7 lead 49 seconds into the third quarter.

Brian Westbrook had six catches for 87 yards and a touchdown for San Francisco. Vernon Davis had five receptions for 70 yards and a score while Josh Morgan caught three passes for 82 yards and a touchdown.

Anthony Dixon led the 49ers with 60 yards rushing on 14 carries. Jeff Reed added field goals of 33, 44, 22 and 36 yards for San Francisco.

Seattle's Ruvell Martin had four receptions for 73 yards and a touchdown and Deon Butler added five catches for 68 yards and a score. Marshawn Lynch had seven catches for 37 yards and also had 29 yards rushing on 10 carries for the Seahawks.

Leon Washington had 225 return yards for Seattle, including a 92-yard kickoff return for a touchdown in the third quarter.