Sidney Crosby lofted a pass to Guentzel, who skated past Panthers defenseman Mike Matheson before dragging the puck around Roberto Luongo and scoring on a backhand into an open net.
Crosby had three points (one goal, two assists), including his 1,200th in the NHL, and Matt Murray made 32 saves for Pittsburgh (35-22-9), which is 7-2-2 in its past 11 games and holds the first wild card into the Stanley Cup Playoffs from the Eastern Conference, two points ahead of the Columbus Blue Jackets.
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"It's a good number," Crosby said about reaching 1,200 points. "I saw [Guentzel] kind of even with their D (on the game-winner), battling for position. I just tried to put a puck in an area and hope he could skate into it. He did a great job holding off the guy and getting control."
Henrik Borgstrom and Vincent Trocheck scored, Mike Hoffman had two assists, and Luongo made 34 saves for Florida (28-26-12), which has lost five straight (0-1-4) and trails the Montreal Canadiens by nine points for the second wild card into the playoffs from the East.
"It's another hard-fought point," Panthers coach Bob Boughner said. "It's just getting frustrating, obviously, not being able to execute in the overtime, or put a game away in overtime. ... I think that we have to execute better in overtime and bury our chances. To talk about our effort, talk about our battle and our compete, all those things were there. We had a couple turnovers that cost us."