Ruutu's goal through the legs of Pittsburgh goalie Marc-Andre Fleury gave the Senators a 5-3 advantage and the momentum to post the win at Mellon Arena in Pittsburgh.
Brian Elliott made 17 saves in the Sens' win.
The fourth-seeded defending NHL champion Penguins got a pair of goals from Evgeni Malkin in the loss, while Craig Adams and Alex Goligoski also notched Pittsburgh scores.
Fleury was able to stop only 21 of 26 Ottawa shots.
Game 2 is set for Friday in Pittsburgh.
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UPI Sports Results for September 19, 2014