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Marleau signed a three-year, $18.75 million contract with Toronto on July 2 after playing 19 seasons with the San Jose Sharks.
Leo Komarov chipped the puck up the boards to Marleau, who had a partial breakaway before scoring on a wrist shot past Mike Condon at 11:26 of the first period.
James van Riemsdyk also scored for the Maple Leafs. Frederik Andersen allowed two goals on 18 shots in the first two periods, and Kasimir Kaskisuo allowed one goal on four shots in the third.
Erik Burgdoerfer, Fredrik Claesson, Filip Chlapik, Patrick Sieloff and Logan Brown, the No. 11 pick in the 2016 NHL Draft, scored for Ottawa. Condon allowed two goals on 24 shots. Marcus Hogberg made 16 saves in relief.
Video: OTT@TOR: Marleau snipes wrister in Leafs debut
Forbserg helps Blackhawks defeat Blue Jackets, Panarin
Anton Forsberg made 38 saves for the Chicago Blackhawks in a 5-2 win against the Columbus Blue Jackets at Nationwide Arena.
Forsberg was part of the June 23 trade that sent forward Artemi Panarin to Columbus. Panarin had three shots on goal in 22:01.
Nick Schmaltz and Cody Franson each had a goal and an assist, and Laurent Dauphin, Jan Rutta and Vinnie Hinostroza scored for Chicago, which scored four goals in the second period.
Seth Jones scored twice for Columbus. He ended Forsberg's shutout with a goal 5:52 into the third period and had a power-play goal with 3:19 remaining.
Video: CHI@CBJ: Forsberg makes a flurry of saves
Eichel scores in OT to give Sabres win against Penguins
Jack Eichel scored less than a minute into overtime to give the Buffalo Sabres a 4-3 win against the Pittsburgh Penguins at Pegula Ice Arena on the Penn State campus in University Park, Pennsylvania.
Defenseman Matt Tennyson tied it 3-3 with 2:39 remaining after defenseman Olli Maatta scored a power-play goal at 8:12 of the third period to put the Penguins ahead 3-2 in their preseason opener.
Jason Pominville scored two goals for the Sabres. It was his first game back with Buffalo since being acquired in a trade from the Minnesota Wild on June 30; the 34-year-old forward did not play in a 3-2 overtime loss to the Carolina Hurricanes on Monday.
Pominville, who played for the Sabres from 2003-13 after they selected him in the second round (No. 55) of the 2001 NHL Draft, put Buffalo ahead 2-1 at 10:38 of the second period. Center Jay McClement, who is in Pittsburgh training camp on a professional tryout, scored 2:34 into the third period to tie it 2-2.
Pominville gave the Sabres a 1-0 lead 7:19 into the game, but Penguins forward prospect Zachary Aston-Reese tied it 1-1 less than a minute into the second period.
Purcell scores in Bruins' win; Krug injured
Teddy Purcell, in Boston Bruins camp on a professional tryout, scored in a 4-2 win against the Detroit Red Wings at TD Garden.
Bruins defenseman Torey Krug left the game after he was hit in the face by a deflected puck.
Ryan Fitzgerald, Danton Heinen and Austin Czarnik scored second-period goals to give Boston a 3-0 lead. Purcell made it 4-2 with 2:46 remaining in the third. Anton Khudobin made 20 saves.
Anthony Mantha had a goal and an assist, and Dylan Larkin scored for Detroit. Jared Coreau made 11 saves, and Petr Mrazek made 19; each allowed two goals splitting the game for the Red Wings.
Benn helps Stars come back to defeat Blues
Jamie Benn scored the go-ahead goal on the power play 12:19 into the third period to help the Dallas Stars to a 5-3 win against the St. Louis Blues at American Airlines Center.
Dallas forward Brett Ritchie scored to tie it 3-3 at 8:04 of the third. Ben Bishop allowed three goals on 15 shots in his Stars debut, and Mike McKenna made 11 saves in relief.
Tyler Pitlick scored an empty-net goal for Dallas with 1:44 remaining in the preseason opener for each team.
Mattias Janmark scored at 11:47 of the first period to get the Stars within 2-1 after the Blues got goals from Wade Megan (3:19) and Klim Kostin (11:14). Nicholas Caamano tied it 2-2 at 4:11 of the second.
Samuel Blais scored at 12:15 of the second to give St. Louis a 3-2 lead. Blues starting goalie Carter Hutton allowed two goals on 11 shots, and Jordan Binnington made 10 saves.
Power play helps Hurricanes defeat Lightning
Justin Williams and Teuvo Teravainen scored power-play goals to help the Carolina Hurricanes to a 2-1 win against the Tampa Bay Lightning at Amalie Arena.
Williams opened the scoring 3:46 into the game. Teravainen made it 2-0 at 4:03 of the second period.
Alex Nedeljkovic started in goal for Carolina and made 14 saves through two periods. Callum Booth made 11 saves in the third.
Lightning forward Adam Erne scored a power-play goal at 15:37 of the second period to make it 2-1.
The Lightning thought they tied the game late in the third period on a goal by Alexander Volkov, but it was waved off because of goalie interference.
Michael Leighton made 20 saves for Tampa Bay.
Brickley scores twice for Panthers in win against Predators
Forward Connor Brickley had two goals and an assist to help the Florida Panthers to a 5-3 win against the Nashville Predators at Bridgestone Arena.
Brickley, a second-round pick (No. 50) by the Panthers at the 2010 NHL Draft, gave them a 2-0 lead at 9:28 of the second period and made it 3-2 1:43 into the third.
Henrik Haapala, Michael Matheson and Denis Malgin also had goals for Florida. James Reimer stopped all 15 shots he faced; Harri Sateri replaced him midway through the second period and made 16 saves.
Cody McLeod and Emil Pettersson scored 5:02 apart in the second period for the Predators. Viktor Arvidsson scored a power-play goal at 11:23 of the third to get them within a goal at 4-3 before Malgin scored into an empty net at 19:31.
Juuse Saros made 23 saves for Nashville.
Salomaki's OT goal lifts Predators past Panthers
Miikka Salomaki scored his second goal of the game at 3:07 of overtime to give the Nashville Predators a 3-2 win against the Florida Panthers at Bridgestone Arena.
Colton Sissons also scored for the Predators. Pekka Rinne started in goal and made 10 saves. Anders Lindback relieved him midway through the game and stopped 14 of 15 shots.
Vincent Trocheck and Jared McCann scored for the Panthers. Roberto Luongo started and made eight saves before being replaced by Samuel Montembeault, who allowed three goals on 11 shots.
Sissons gave the Predators a 2-1 lead with 1:11 remaining in the second period. McCann tied it at 12:49 of the third.
Fast start helps Golden Knights top Avalanche
Brad Hunt, Jonathan Marchessault and Oscar Lindberg scored in a span of 2:31 in the first period to help the Vegas Golden Knights to a 4-1 win against the Colorado Avalanche at Pepsi Center.
Maxime Lagace started for the Golden Knights and stopped 10 of 11 shots. Oscar Dansk made 18 saves in the third period.
Hunt scored at even strength 5:16 into the game. Marchessault (6:09) and Lindberg (7:47) each scored on the power play.
Tomas Hyka gave Vegas a 4-1 lead at 15:06 of the third period.
Nathan MacKinnon scored a power-play goal for the Avalanche at 17:03 of the second period. Spencer Martin made 24 saves in Colorado's first preseason game.
Video: VGK@COL: Marchessault beats Martin five-hole
Dell, Sharks shut out Ducks
Aaron Dell made 24 saves, and Daniel O'Regan, Mikkel Boedker, Kevin Labanc and Brandon Bollig scored in the second period of a 5-0 win for the San Jose Sharks against the Anaheim Ducks in the preseason opener for each team at SAP Center.
Filip Sandberg scored in the first period. Barclay Goodrow and Tim Heed each had two assists.
Reto Berra made 31 saves for the Ducks.