San Francisco • On a night when San Francisco's offensive stars sat out, the 49ers got more big plays on the other side of the ball to cap their first unbeaten preseason in 18 years.

Manny Lawson and rookie Taylor Mays each had interceptions and rookie Anthony Dixon ran 46 yards for a first-half touchdown in the Niners' 17-14 victory over the San Diego Chargers on Thursday night.

Jehuu Caulcrick caught a 2-yard touchdown pass from Nate Davis with 8:41 left and Joe Nedney added a 25-yard field goal. San Francisco (4-0) went undefeated in its exhibition schedule for the first time since a 5-0 showing in 1992.

Jeremy Williams returned a punt 93 yards for a touchdown, then caught a 29-yard pass to set up Curtis Brinkley's 2-yard TD run late in the third quarter for the Chargers. But Curtis Taylor stripped Williams after a catch to force a fumble and give San Francisco the ball back with 6:54 to play.

Williams also was the intended receiver on San Diego's last-ditch effort on fourth down in the final minute. Tramaine Brock picked off Jonathan Crompton's pass with 48 seconds left.

San Francisco's offense looked little like the unit it will be for the team's Sept. 12 opener at NFC West rival Seattle. No former Ute Alex Smith at quarterback, no Frank Gore or Brian Westbrook at running back, no Michael Crabtree at wideout or Joe Staley leading the line at left tackle.

Chargers quarterback Philip Rivers also got the night off.

The Niners did go with their regular starting on 11 on defense — but for one series.

Lawson blitzed untouched toward Billy Volek on San Diego's opening drive of the second quarter, batted the ball with both hands and turned to catch it for an interception.

"Man, first of all, I got to give a shout out to my DBs and other linebackers for covering and allowing me time to get back there," Lawson said. "I was just fortunate enough to, I got my hand in the way and tipped the ball and it floated in the air. All it was after that was just catching it." —

In short

R San Francisco beat San Diego 17-14 to cap their first unbeaten preseason in 18 years.