"It's just one game. I try to keep it in perspective," Manning said. "I know how hard I've worked to get to this point. I know how much help that I've gotten along the way, from a lot of people. I'm very grateful for that. It's special. I will definitely say it's special."
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The four-time MVP coming off four neck surgeries that wiped out his final season in Indianapolis last year completed 19 of 26 passes for 253 yards and two touchdowns. He was sacked twice and bounced up both times without trouble.
Manning got a big assist from another big-time free agent, cornerback Tracy Porter, whose 43-yard interception return of Roethlisberger's pass with two minutes left and the Broncos clinging to a six-point lead sealed the victory.
"I should've called timeout," Roethlisberger said. "The play clock was running down, I hate to burn timeouts. I should've. We were kind of all over the place.
"There's no one to blame but myself. I already told my teammates and coaches that it's my fault. It's on me. It's disappointing. I hate to let my team down, my coaches, the fans. That loss is squarely on my shoulders. I'll take that and get better and learn from it."
Manning, jettisoned by the Colts in March after missing all of last season with a nerve injury that weakened his throwing arm, took only two snaps in the third quarter. But one was a 71-yard touchdown throw to Demaryius Thomas, Manning's first as a Bronco and the 400th of his career.
He reached the milestone in his 208th career game. Dan Marino did it in his 227th game and Brett Favre in his 228th.
"I love those guys," Manning said. "Brett and Dan are two of my favorite quarterbacks of all time. I don't feel like I belong in their category. But to be sort of mentioned among them is a true honor."
Manning, however, didn't even keep the ball for his trophy case.
"I talked to him after the game and said, 'You're going to let me keep the ball right?' He got one of the game balls, so he gave me that ball," Thomas said. "That's great to be able to catch his 400th. Hopefully he'll sign it for me. I can go back and show my grandkids in the record book."
Manning, who hadn't played in a game that counted since Jan. 8, 2011, spent much of his night on the sideline as Roethlisberger led a clock-chewing offense that had Denver's defenders gassed and Manning antsy as the Steelers converted 11 of 19 third downs.
"That's what Peyton kept saying to me: 'You guys killed us on third down.' We did," Roethlisberger said. "But at the end of the day it doesn't matter when you lose the game."
At one point, Roethlisberger had taken 30 snaps to Manning's one -- a kneel-down -- during two monstrous drives that spanned halftime.
"Roethlisberger was awesome on third down," Manning said. "Wasn't much fun sitting on the bench there all night. Great job by our defense in the end, getting that turnover."
Steelers coach Mike Tomlin said playing keep-away from Manning was the game plan.
"That was our intention," Tomlin said. "We need to possess the ball, we needed to get stops and time of possession is just that, though you look at the scoreboard, they got it done, we didn't. The plan didn't come to fruition."
Whenever he was between the lines, Manning was masterful.
After going to the no-huddle midway through the first half, he led Denver on three consecutive 80-yard touchdown drives, picking apart Pittsburgh's defense with precision.
"It helped a lot," Manning said of the hurry-up. "We're going to use the no-huddle as a change-up. Once we did it, it went so well, the coaches kind of said stay with it. I was kind of afraid that might happen."
So were the Steelers.
The Broncos' last drive stalled at the Steelers 8 and they settled for Matt Prater's short field goal and a 25-19 lead with three minutes left.
That was more than enough time for Roethlisberger to beat them, Manning figured.
Porter, though, stepped in front of Emmanuel Sanders and returned the interception 43 yards for the score, and Roethlisberger was sacked three times on his last possession after going down only twice all night.
"Better late than never," said Von Miller, who had two of those sacks.
"That was a great feeling," Manning said. "I hated having to settle for a field goal down there. ... I wanted to get a touchdown, make it a two-score game. You don't like giving the ball to Ben on a two-minute drive. Our defense had been bending all game but never quite breaking. That was a huge play by Porter. Von was bringing the pass rush at the end there, that was fun to see."
Manning's 71-yard TD to Thomas gave Denver a 14-13 lead, but the drive took all of 36 seconds, and the Steelers had possession for the rest of the third quarter, running 27 plays against a gassed Denver defense.
Although this one was a screen pass, Thomas' long TD was reminiscent of his 80-yard touchdown on a crossing pattern on the first play of overtime that beat the Steelers 29-23 in the AFC wild-card game eight months ago when Denver was Tim Tebow's turf.
That changed when Manning was released by the Colts on March 7 after 14 seasons and chose Denver as his destination for his comeback two weeks later after a frenzied free agency tour. The Broncos sent Tebow to Broadway 24 hours later.
Manning looked as good as ever Sunday night.
"What can you say?" Steelers safety Troy Polamalu said. "I mean, he's Peyton Manning. He's the same Manning. Everything anybody has ever said about him is probably the same thing I would say tonight."
- Denver DT Ty Warren, playing in his first game since 2009, sprained an elbow in the first half and didn't return.
- Also hurt in the first half were Steelers O-linemen Marcus Gilbert (knee) and Ramon Foster (eye).
- Steelers star LB James Harrison (knee) sat out, as did S Ryan Clark, who has sickle cell trait that makes playing at altitude dangerous.