INDIANAPOLIS – After insisting that the preseason had no bearing on how they would play when the games started to count in the standings, the Bears went out and proved it Sunday night in Indianapolis.

Backed by an aggressive, big-play defense and an efficient offense fueled by a power running game, the Bears shocked the Colts 29-13 in the regular season opener at Lucas Oil Stadium.

Adewale Ogunleye (93) celebrates after tackling Joseph Addai for a safety in the Bears' 29-13 win over the Colts Sunday night in Indianapolis.

“I’ve said all along you get to your first game to see where you are,” said coach Lovie Smith. “Preseason [games], they don’t count. You want to go out and practice hard for every game. But behind the scenes, you’re trying to do everything you can to get ready for this first game.”

Playing like the unit that led the Bears to the Super Bowl in 2006, the defense produced a touchdown on Lance Briggs’ 21-yard fumble return, registered a safety and made a key fourth-down stop at midfield with the Bears protecting a 22-13 lead early in the fourth quarter.

Offensively, rookie Matt Forte staked the Bears to a 7-3 lead with a 50-yard touchdown run and rushed for 100 of his game-high 123 yards in the first half. Kyle Orton made several clutch throws while quarterbacking an offense that produced 183 yards rushing, converted 10 of 16 third-down opportunities and didn’t commit a turnover.

In stunning one of the NFL’s premier teams, the underdog Bears nearly reversed the score from Super Bowl XLI, when they lost to the Colts 29-17 in Miami.

“We expected to win,” middle linebacker Brian Urlacher said Sunday night. “It’s not an upset to us. We practiced hard the last two weeks getting ready for this game. We’re a good football team.”

The Bears played virtually the entire game in their nickel defense with Urlacher and Briggs on the line of scrimmage where they could blitz quarterback Peyton Manning or drop into coverage.

“You don’t see a lot of teams do that a whole game against Peyton and the Colts, but we did,” Briggs said. “We believed in our scheme. We believed in our game plan and we stuck to it. That’s what we do best. We fly around, get to the ball, create turnovers and get in the end zone.”

Briggs got in the end zone late in the third quarter after Indianapolis had cut the Bears’ lead to 15-13 on Manning’s 6-yard TD pass to Reggie Wayne. Charles Tillman stripped the ball from Marvin Harrison at the Colts’ 21 after a 5-yard reception, and Briggs scooped up the fumble and dashed into the end zone to widen the margin to 22-13.

“He left it out there and I just tried to swat it,” Tillman said. “I didn’t know Lance could run like that. He did a great job, and so did the rest of the defense blocking for him.”

Indianapolis responded by marching to the 50-yard line. But on fourth-and-one, Briggs and Adewale Ogunleye teamed up to stuff Dominic Rhodes for a two-yard loss.

Orton lofted a 26-yard completion to tight end Desmond Clark on third-and-one to the Colts’ 1, and fullback Jason McKie capped the 8-play, 48-yard drive with a 1-yard TD plunge two plays later to give the Bears a 29-13 lead with 8:56 remaining in the game.

Orton completed 13 of 21 passes for 150 yards with no touchdowns, no interceptions and an 83.4 passer rating that was slightly better than his Colts counterpart. Manning connected on 30 of 49 passes for 257 yards with 1 TD, no interceptions and an 81.8 passer rating.

“I was very impressed [with Orton],” said offensive coordinator Ron Turner. “He showed great poise and composure. He’s very even-keeled. He always is. He’s always in control of what he’s doing and he’s a very smart player.”

After Adam Vinatieri’s 39-yard field goal had given the Colts a 3-0 advantage, Forte gave the Bears a lead they would never relinquish with a 50-yard TD run. Dashing up the middle, Forte cut back brilliantly to avoid safety Antoine Bethea and outraced safety Bob Sanders to the end zone. Guard Roberto Garza buried a Colts lineman to spring Forte.

By halftime, Forte had rushed for more yards than any Bears rookie in his NFL debut.

“He did an unbelievable job for his first game,” Orton said. “He made a cut going to the right and then back to the left that was big league. He played great and didn’t seem fazed at all.”

After Vinatieri’s 34-yard field goal had drawn the Colts to within 7-6, Robbie Gould answered with a 41-yarder that gave the Bears a 10-6 lead with 4:43 left in the first half.

Chicago’s defense then asserted itself on back-to-back plays. After rookie defensive tackle Marcus Harrison sacked Manning for a 10-yard loss back to the Indianapolis 2, Ogunleye dropped Joseph Addai for a two-yard loss and a safety, widening the margin to 12-6.

The Bears defense held the Colts running game to just 53 yards on 15 carries.

“They played hard the entire time,” Smith said. “That’s what we’re used to seeing.”

After Gould’s 25-yard field goal on the final play of the first half, the Bears survived a near disaster on the second half kickoff. Devin Hester caught the ball seven yards deep in the end zone, stood still for a few seconds trying to lull the Colts to sleep and then took off. But Melvin Bullitt wasn’t fooled, and he tackled Hester at his own 3-yard line.

The Bears failed to pick up a first down, but Brad Maynard boomed a 55-yard punt. The Colts scored their only touchdown on the ensuing possession, but the Bears answered again and again.