"The season is short and our power plays are going to be important," said defenseman Andrej Meszaros, adding that the Flyers did too much passing and not enough shooting with an extra attacker.
In franchise history, the Flyers are 21-17-7 in season openers.
Trailing 2-1, the Flyers got their fourth power play with 5:33 left. They had a handful of scoring chances, but goalie Marc-Andre Fleury, who struggled mightily in last year's playoff loss to the Flyers, had all the answers.
With 2:27 to go, the Penguins' Evgeni Malkin went to the penalty box for high-sticking. The Flyers pulled goalie Ila Bryzgalov (24 saves) with about 1:30 left and had a 6-on-4 advantage. But that was quickly negated by Giroux's tripping penalty with 1:02 to play.
Chris Kunitz iced the win with an empty-net, power-play goal with 11 seconds left.
The Flyers, soundly outplayed in the first period, got to within 2-1 as Giroux, in his first game as captain, converted a slick saucer pass from Hartnell and tapped it past Fleury 23 seconds into the second period. Hartnell's pass somehow made it past Kunitz and onto Giroux's stick.
The Penguins built a 2-0 lead on first-period goals by Tyler Kennedy (power play) and James Neal. Kennedy scored on a tip-in that may have deflected off Flyers defenseman Luke Schenn, while Neal scored after Malkin won a draw and fed him for a quick shot from the left circle.
From there, the Penguins did just enough things right to survive.
"We knew it was going to be a little scrambly, a little sloppy. Everyone's still trying to get their timing down," Neal said. "It's been a long time since we've been in a game situation. But no better way to start the season off than with a big win in Philly."
After a sluggish first period in which they had as many giveaways (six) as shots, the Flyers dominated on the forecheck and increased their physicality in the second period.
"From the second period on, I thought we were the better team," Meszaros said. "We have to make sure we have a good first 10 minutes and go from there."
The Penguins scored the two goals in the first 7:20, then held off the Flyers because of their penalty-killing units. They quieted a crowd that still managed several derogatory chants directed at Penguins star Sidney Crosby, who won 65 percent of his face-offs and was minus-1.
"You're pretty focused on the game and you don't usually hear that too much," he said of the chants. "Hear it a little bit more when you lose, that's for sure. It's always a little bit easier when you win."
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