INDIANAPOLIS — The Indianapolis Colts proved Sunday they are capable of a letdown.

It remains to be seen whether the team is capable of a loss.

Running back Joseph Addai threw a 22-yard touchdown pass to Reggie Wayne on the first snap of the fourth quarter, and the Colts (7-0) gutted out an 18-14 victory against the San Francisco 49ers (3-4) at Lucas Oil Stadium.

“It was one of those days, we kinda call them grinders,” Indianapolis quarterback Peyton Manning said. “You just have to keep grinding it out. Fortunately, our defense was able to keep them from scoring a lot, and we were able to make some plays at the end. You have to give San Francisco a lot of credit.”

The 49ers held Manning without a touchdown pass, but he did complete 31 of 48 attempts for 347 yards — his sixth game with at least 300 yards this season and the 53rd of his storied career.

But after a string of dominant victories, the outcome against the 49ers was constantly in doubt.

San Francisco struck first on a 64-yard touchdown run by Frank Gore at the 11:16 mark in the first quarter. The Colts answered with field goals from Matt Stover late in the first quarter and early in the second.

Then the defenses settled in.

Manning looked rusty in the first half, even though he connected on 15 of 24 passes for 175 yards.

There were at least two occasions when a Colts receiver broke free for a potential big play, and Manning simply missed him.

“I’m sure we’ll see some things on film,” he said. “We didn’t have much of a rhythm out there.”

San Francisco took advantage of the Colts’ stumbling late in the half.

Quarterback Alex Smith directed a seven-play, 89-yard drive that took just 1:11 off the clock and gave the Niners a 14-6 lead. Smith hit on seven of eight passing attempts on the drive, and San Francisco also got the benefit of a roughing the passer penalty at Indianapolis’ 25-yard line.

That gave the Niners a first down at the Colts’ 12, and Smith quickly found Gore for four yards. A surgically-precise eight-yard strike to Vernon Davis in the end zone followed for the touchdown.

“I thought the offense came out and did a decent job,” 49ers coach Mike Singletary said. “But you can’t have any missed opportunities. In a game like this, that will bite you in the end, and it did.”

Singletary was referring to couple of dropped passes in the second half and one fumble the Colts recovered but failed to capitalize on.

Manning drove Indianapolis to San Francisco’s 13 with six seconds remaining in the half, but the Colts settled for another Stover field goal rather than take a shot at the end zone.

“Six seconds, that’s kind of the cutoff point,” Manning said. “Even a routine play can potentially take six seconds.”

Stover added his fourth and final field goal of the day to cap the first drive of the fourth quarter and cut the Colts’ deficit to 14-12.

The score remained there until Manning and company regained possession with 3:03 to play in the quarter.

Starting from his own 30, Manning followed an incompletion with a five-yard pass to Pierre Garcon on the right side. An 11-yarder to Wayne on the left followed and gave Indianapolis a first down at its own 46.

Manning then found Dallas Clark for nine yards to break into San Francisco territory, and Addai rumbled for three yards off left tackle for a first down at the Niners’ 45. A deep pass to Hank Baskett was broken up, and Manning hit Austin Collie for five yards on second down.

A neutral zone infraction gave the Colts a first down at the San Francisco 32, and Addai broke free for 10 yards as the quarter came to a close.

Starting the fourth quarter just outside the red zone, Colts coach Jim Caldwell decided to roll the dice.

Manning handed off to Addai and rolled with his back to the play as he would on any other run.

“You try to do everything you need to do on a normal run play,” he said.

Wayne faked a block in front of Addai, drawing in both the safety and cornerback on his side. When Addai stopped cold and threw, Wayne was well clear of the defense. Wayne’s diving catch gave the Colts the victory.

“We have a pretty good arsenal of things that we use,” Caldwell said. “That one we’ve been working on for a long time. The situation was right, and he did a nice job of throwing it, and Reggie did a great job of catching it as well.”

Singletary admitted the play caught San Francisco’s defense by surprise.

“Yes, it did,” he said. “(Cornerback) Shantae Spencer had been playing well all day. The defensive backs on the back end all day had been very disciplined knowing that they were going to do some play action and things like that. They got us on that one.”

A 2-point conversion failed, but the Colts’ defense made the four-point lead stand up.

Robert Mathis and Dwight Freeney sacked Smith on the final play of the 49ers’ last two drives, and Manning and the offense ran off the final 5:45.

Billboards in San Francisco proclaim Singletary is “not a moral victory kind of guy.” The coach lived up to that statement.

“We are young, and we have to learn how to win those games,” he said. “We have to learn how to finish those games. That is all it says.”

Caldwell became the first rookie head coach to start a season with seven straight wins since Buffalo’s Barry Lepper in 1920. He praised San Francisco’s play and was satisfied with Indianapolis’ response.

“It was a good, tough, hard-nosed battle,” he said. “Singletary certainly had his team ready to play, and they played hard, and they played well. They certainly had a good game plan against us. You find a way to win it. I think our guys hung in there, came through and made some big plays and certainly performed well down the stretch.”