"This was a huge win for us," said Niittymaki, who lost a 13-round shootout to the Rangers at home on Saturday. He bounced back quickly after allowing Marcel Hossa to win the shootout with the only goal in the lengthy tiebreaker.
The Flyers returned the favor and handed the Rangers their first loss (2-1).
"We wanted to show that we wanted to win tonight," Forsberg said. "We know these are big games. We have to battle these guys all year."
New Rangers forward Brendan Shanahan scored his third of the season -- all in New York's two home games -- with 29 seconds left in the middle period. He fired in a hard slap shot off a nice feed from Michael Nylander during a power play to close the gap to 4-2.
It was the first power-play goal of the season for the Rangers.
"Obviously, our effort wasn't good enough and we know that," Shanahan said. "They came out as a team that deserved to win and expected to win. We came out like a team waiting to see what they were going to do. We were spectators the first two periods."
Philadelphia scored twice within 59 seconds of the second period to take a 3-1 lead. Mike Knuble scored his first at 6:59, skating out unchecked from the right corner and deking Lundqvist on a backhander from close range. Geoff Sanderson followed at 7:58, scoring on a 2-on-1 break for his second of the season.
The Flyers came out aggressively at the start, swarming on the forecheck and forcing Lundqvist to make several difficult saves. Forsberg skated out of the right corner along the goal line and scored his second goal on a high, sharp-angle wrist shot from close in at 3:15 to give Philadelphia a 1-0 lead.
"We fought the same way we finished off the last half of the game in Philadelphia," Flyers coach Ken Hitchcock said. "You're seeing our speed up front really creating a lot of problems for teams. Our defense is getting better and better as a group. We're moving the puck better. We're in sync with each other and we're playing hard."
Rangers defenseman Thomas Pock scored his first at 16:33 to make it 1-1. Petr Prucha started the play by blocking a point shot in the New York zone and sending a nifty backhand pass off the sideboard as he fell to the ice to create a 2-on-1 break.
Shanahan skated down the left wing and sent a cross ice pass to Pock, who waited until Niittymaki went down and wristed the puck into the top of the net.
- David Wright, Tom Glavine, Paul LoDuca and Chris Woodward of the New York Mets were in attendance and received an ovation from the crowd. They open the National League championship series at home against St. Louis on Wednesday night.
- The Rangers were 0-for-12 on the power play for the season and 0-for-3 in 5-on-3 advantages before Shanahan's goal.
- Shanahan tied Jari Kurri for 14th on the NHL career goal list with 601.
- Jaromir Jagr, who assisted on Shanahan's tally, has 33 goals and 60 assists in 71 games against the Flyers.
- Forsberg has five goals and 26 assists in 22 career games versus the Rangers.