PHILADELPHIA (AP) -A fast start by the Philadelphia Flyers gave way to a very nervous ending.

When it was over, they were still alive and happily facing what many figured was an unlikely return trip to Pittsburgh.

Joffrey Lupul scored the first of Philadelphia's three first-period goals Thursday night and the Flyers extended the Eastern Conference finals for at least one more game with a 4-2 victory over the Penguins.

''We have the momentum right now, but we have to come out in Pittsburgh like we did tonight,'' forward Mike Richards said. ''We still have a desperate situation here. We know the mountain that we have to climb. As long as we take it slow and not try to win it in the first 10 minutes and play a full 60 we could have success.''

The Penguins' smooth ride toward the Stanley Cup finals got a bit bumpier and will now include a Game 5 on Sunday.

Just as they did after taking a 3-0 lead over the New York Rangers in the second round, the Penguins were beaten in Game 4. Pittsburgh wrapped up that series at home in its next chance, shaking off its only other loss in the playoffs (11-2).

Jordan Staal scored twice in the third period to get the Penguins close, but Lupul sealed the Flyers' first win of the series with an empty-netter in the final minute.

''They had a good first period, and we didn't,'' Penguins captain Sidney Crosby said. ''That was really the difference. We fought hard in the second and third ... but it was just too little, too late.''

The Flyers followed the lead set Wednesday by the Dallas Stars , who stayed alive in the West finals by avoiding a sweep against Detroit.

This is all new for Philadelphia, which had been swept the five other times it trailed 3-0.

The Penguins are 7-0 in the playoffs at home and have won 15 straight. They defeated the Flyers 4-2 there in the first two games of the series.

''We want to finish it off,'' Crosby said. ''I don't think you go into every series expecting to win four in a row.''

Only two NHL teams have recovered from an 0-3 deficit to win, the 1942 Toronto Maple Leafs and the 1975 New York Islanders , who rallied to beat the Penguins. Pittsburgh has led all three series this year 3-0, including a first-round sweep of Ottawa.

Richards and Crosby each earned roughing and slashing penalties and were sent to the dressing rooms before the final seconds ticked off.

''I think he thought I was trying to slash him, but I was just trying to move the puck forward,'' Richards said. ''He was probably angry that he wasn't winning the hockey game and he tried to slash me. We can't worry about getting undisciplined and running around like village idiots.''

Lupul got things going for the Flyers 8:27 in, and Danny Briere and Jeff Carter scored power-play goals 7:02 apart to make it 3-0 in the first.

Martin Biron showed the form that backstopped series wins over Washington and top-seeded Montreal in making 36 saves. Marc-Andre Fleury stopped 30 shots for the Penguins.

Staal, who returned to the Penguins following a one-day absence after the death of his grandfather, spoiled Biron's shutout bid with 16:44 left. He scored again with 5:49 remaining to transform Wachovia Center from joyous to nervous.

After trailing 11-4 on the shot clock midway through the first, the Flyers broke out against the previously sharp Fleury and finished the frame with a 17-13 edge.

In Tuesday's 4-1 loss in Game 3, Philadelphia managed only eight shots through two periods. The Flyers had 26 in the first 40 minutes this time.

Once they got the ever-elusive lead, the Flyers looked a little like the Penguins as they tried to clog up the neutral zone.

Pittsburgh has been doing that to perfection, allowing only 22 goals before Thursday.

In a period the Flyers had been waiting four games to unleash, Philadelphia scored three times to take control for the first time in the series. After netting five goals in falling into an 0-3 hole, the Flyers struck for three in a span of 10:23.

Not bad for a club that hadn't been in front since the first period of Game 1, a lead that lasted only 1:21.

''It's always nice to score first,'' Richards said. ''It allows us to play a little looser, and instead of playing from behind do things that we've had success doing.''

After going 1-for-9 on the power play in the first three games, the unit that was the NHL's second-best during the regular season started to click. It took time for the Flyers to overcome the loss of top defensemen Kimmo Timonen and Braydon Coburn , who are out with injuries but could be back Sunday.

Randy Jones fired a shot as he glided along the blue line, and the puck bounced off Fleury into the slot. Briere squeezed his way past 6-foot-7 defenseman Hal Gill and swept in a rebound from his knees at 11:48.

Carter also scored from in close, banging away at a rebound of R.J. Umberger's shot and finally knocking it past Fleury with 1:10 left in the period.

Philadelphia was successful in its first game with new lines. In an attempt to spark the offense, coach John Stevens moved Richards up to play with Briere and dropped down slumping forward Vinny Prospal.

Notes: The Penguins were held without a power-play goal for just the third time in the playoffs. They are 1-2 in those games. ... Coburn wasn't healed enough from a swollen eye to return, but might return soon. Timonen will skate Friday and could also make a surprise comeback. ... Penguins LW Gary Roberts missed his second straight game with pneumonia. He is day to day.