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INDIANAPOLIS -- Colts quarterback Peyton Manning made things look easy Sunday. Sixteen plays, 10 points, take a breather.

Everyone else found out how tough the NFL really can be when you're not the four-time league MVP -- even if it is only the preseason.

Running back Michael Robinson fumbled on the 49ers first play, David Carr was sacked three times and the San Francisco receivers struggled to hold onto Alex Smith's passes.

Fortunately for San Francisco, Manning's backup, Curtis Painter, played even worse. The 49ers took advantage of Painter's three interceptions and a botched handoff late in the first half, scoring 34 straight points to rally for a 37-17 victory at Indy.

"I think he didn't play as well as he'd like to, but he needed some help, too," Colts coach Jim Caldwell said of Painter. "The things that are hidden, dropped passes, the things you need to extend drives, you need guys to do their jobs and we certainly didn't play as well as we wanted."

The game did rekindle some old memories, though.

Manning was in midseason form, going 8 of 10 for 91 yards and leading the Colts to scores on their first two drives just minutes after accepting the MVP trophy.

Smith, who finally emerged last season as the quarterback the 49ers expected, was 3 of 9 for 37 yards and managed only two first downs.

Carr, like Manning and Smith was a No. 1 pick in the draft, finished 9 of 11 for 98 yards with one TD, but was sacked three times and nearly a couple of more -- reminiscent of his early days in Houston.

Then there was Painter, whom Colts fans booed mercilessly after he made his NFL debut in December's loss to the Jets. He was 9 of 19, recovered one fumble, had one tackle and endured even more boos from the half-full stadium just before halftime.

The 49ers actually had more return yards on Painter's interceptions (111) than Painter threw for (64), and, as Caldwell, noted, it wasn't all Painter's fault.

"I thought we were tough physically and mentally and we ran to the ball," 49ers coach Mike Singletary said. "I thought the guys did a good job overall."

Clearly, this was not what the Colts wanted to see.

Then again, Indy has never made preseason games a priority.

Six starters including Pro Bowlers Dallas Clark, Dwight Freeney, Bob Sanders and Jeff Saturday -- did not play on Sunday.

Coach Jim Caldwell yanked most of his other starters, along with Manning, after just two series. And the running game Indy wants to improve produced only 8 yards in the first 25 minutes.

San Francisco wasn't at full strength, either. Michael Crabtree, Frank Gore and Patrick Willis all sat out, but it wanted to use the game to get in sync.

The 49ers struggled early. Robinson's fumble led to Adam Vinatieri's 21-yard field goal, and Manning took the Colts 89 yards on the next series -- the last 7 coming on Joseph Addai's scoring run midway through the first quarter.

"I thought our guys, really, after a slow start, got back on track," Singletary said. "Overall, I thought the guys did a pretty decent job. We're on the right road, but we have a long way to go."

That's when Painter entered and the Colts lead vanished.

Reggie Smith picked off Painter on the left sideline and ran it back 91 yards for a TD to make it 10-7.

"I saw an opportunity," Smith said. "They were running two short routes and one deep, and the ball bounced up in the air. Right place, right time, came and got it and took it the rest of the way."

Kyle Williams used the same philosophy when he caught Pat McAfee's low punt and ran it back 36 yards to set up Robinson's 3-yard scoring run. Suddenly, with 7:21 to go in the half, the 49ers led 14-10.

It was only the start.

Before the half ended, Painter threw two more interceptions and ran into fullback Mitch King, losing the ball on a fourth-down play at the Indy 41. San Francisco turned that into two field goals for a 20-10 lead, and Carr sealed the outcome with an 8-yard TD pass to tight end Tony Curtis late in the third quarter.

Indy didn't score again until Tom Brandstater threw a 3-yard TD pass to Jacob Tamme to make it 34-17 with 10:58 left.

"Obviously, the big thing was turnovers," Caldwell said. "We've got a lot of work to do, so we'll keep working."

Game notes

Robinson scored, but it was Anthony Dixon who had the best day of the 49ers running backs. Dixon ran 32 times for 100 yards and had a TD as the 49ers look for a replacement to the recently retired Glen Coffee. ... Four 49ers were injured during the game. Singletary said tight end Vernon Davis has a strained knee, defensive end Derek Walker a bruised hip, cornerback William James a sprained ankle and Williams a sprained toe. ... The Colts lost two players to injuries. Safety Jamie Silva went to the locker room on a golf cart after spraining his left knee and starting guard Kyle DeVan hurt a hamstring.