SAN FRANCISCO (AP) -Alex Smith showed he can deliver a crushing block and bring down a ball carrier with a tackle.

Unfortunately for Smith, those tasks are secondary when it comes to being an NFL quarterback. It was when he was called upon to pass that Smith struggled.

Smith completed 3 of 9 passes and threw an interception in what might have been his last chance to beat out Shaun Hill for San Francisco's starting quarterback job before the 49ers held off the Oakland Raiders 21-20 Saturday night.

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Hill, who started the exhibition opener, wasn't a whole lot better, going 3 for 7 for 20 yards. Coach Mike Singletary would like to pick a starter before next week's exhibition game in Dallas, putting more importance on this game.

``I wanted to see both of them do more in the time they were in,' Singletary said.

Rookie Nate Davis led the Niners to three scores in the fourth quarter, with Kory Sheets' 5-yard run giving San Francisco a 21-14 lead. Charlie Frye scored on a 14-yard run for Oakland but threw an incompletion on the 2-point conversion attempt with 3:30 to go.

The game came the same week that coach Tom Cable was accused of breaking assistant Randy Hanson's jaw in an attack that is being investigated by the police and the NFL.

Smith, the No. 1 overall pick in the 2005 draft, is running out of time to convince Singletary that he should be the starter this season. Expected to be San Francisco's franchise quarterback, Smith instead has struggled when healthy before missing much of 2007 and all of last season with an injured shoulder.

He completed a 15-yard pass to Vernon Davis on his first drive before it stalled and then made his best play on the second. On a reverse to Arnaz Battle, Smith lowered his surgically repaired right shoulder into defensive end Greg Ellis, knocking him to the turf. Battle lost 6 yards on the play, but Smith drew cheers for his block.

``It felt good,' he said. ``As quarterbacks you don't get to deliver many blows. It was nice to be able to do that.'

Two plays later, Smith threw high to Josh Morgan and the ball deflected into Ricky Brown's hands. Brown returned the ball 47 yards before being tackled by Smith at the San Francisco 7.

Smith threw only one more pass before leaving after the first quarter.

``The numbers would say last week was better but I felt much better this week,' Smith said. ``I think numbers can be deceiving.'

Hill didn't get many chances to throw as the Niners relied heavily on rookie running back Glen Coffee.

Coffee, a third-round pick out of Alabama, gashed Oakland's run defense, ripping off four runs of at least 10 yards, including a 35-yarder that led to a field goal by Joe Nedney. Coffee, who replaced Frank Gore after one series, finished with 129 yards on 16 carries.

``I think when you're running it that successfully, why go away from it,' Hill said. ``Sometimes that's how it is.'

Cable was upset with his team's discipline, especially on defense. The 49ers ran for 275 yards against a Raiders team that has spent much of training camp looking to improve a leaky run defense.

``I did not think we tackled real well on defense,' Cable said. ``A couple of times we really had guys in the hole who had to just wrap up and they did not do that.'

Making the situation even worse for Oakland's run defense was that middle linebacker Kirk Morrison left in the first quarter with a dislocated left elbow. Morrison is battling Brown for the starting spot and said he will miss a couple of weeks.

JaMarcus Russell started slowly for Oakland, missing an open Darrius Heyward-Bey on third down of his first drive and tripping over Darren McFadden's feet on third down on the second drive. After Brown's interception, Russell was unable to get Oakland in the end zone, throwing high to Murphy in the back of the end zone.

Russell found his groove on the next drive, starting with a 12-yard pass to Zach Miller. He completed an 11-yard swing pass to Justin Fargas and then got a 22-yard gain when Nate Clements was called for pass interference against Johnnie Lee Higgins.

After a pair of penalties backed Oakland up, Russell took advantage of a miscommunication between Clements and Mark Roman to find a wide open Murphy in the end zone on third-and-21 to end his night. That was Russell's only completion to a wide receiver all night. He finished 7-for-11 for 76 yards.

NOTES: Jeff Garcia made his Oakland debut and threw an interception to end his only drive late in the second quarter. ... Michael Robinson added 97 yards on 14 carries for San Francisco. ... Niners OL Adam Snyder limped off late in first half right ankle and knee sprain that is not believed to be serious.