PITTSBURGH, Pennsylvania (Reuters) - The Ottawa Senators upset Stanley Cup champions Pittsburgh 5-4 on Wednesday to take a 1-0 lead in their first round Eastern Conference series.

Jarkko Ruutu scored the Senators' fifth at 9:40 in the third period to seal the win after the Penguins had clawed back to 4-3 through a goal to Craig Adams.

The Penguins challenged again when Alex Goligoski scored with 2:24 remaining, but were unable to put an equalizer past goaltender Brian Elliott, who needed only 17 saves in his maiden postseason appearance.

"We believe in ourselves and we believe we can take it all the way," Elliott told reporters. "Every win is so meaningful. Especially the first one tonight."

The fifth-seeded Senators fell behind three minutes into the first period then answered with three straight goals from Peter Regin, Chris Neil and Chris Kelly to seize control in the first of their best-of-seven series.

In-form center Evgeni Malkin scored Pittsburgh's first two to trim the deficit to 3-2 midway through the second but Erik Karlsson added a power-play goal to give the Senators breathing space.

In contrast to Elliott, opposing netminder Marc-Andre Fleury was under relative siege from the Senators' offense, stopping 21 of 26 shots.

Maurice Richard trophy co-winner Sidney Crosby tallied three assists for the Penguins, playing their third first round series against the Senators in four years. The Penguins won their last first round series in 2008.

In other Eastern Conference action, the Philadelphia Flyers scored two second period goals to defeat the New Jersey Devils 2-1 and take a 1-0 series lead.

Out West, the Colorado Avalanche finished last in the Western Conference a year ago, but shocked the top-seeded San Jose Sharks 2-1 in their opener, courtesy of a Chris Stewart goal with only 50 seconds left.

Western Conference champions the Detroit Red Wings also fell 3-2 to the Phoenix Coyotes, who came back from 2-1 down after power play goals scored by Wojtek Wolski and Derek Morris in the second and third periods respectively.

(Reporting by Jahmal Corner in Los Angeles; Editing by Ian Ransom)