The Sharks improved to 3-0-0. They have outscored opponents 11-4.
Drake Batherson scored for the Senators.
Brady Tkachuk and goaltender Matt Murray made their season debut for Ottawa. Tkachuk, who signed a new contract last week, assisted on Batherson's goal. Murray made 22 saves after missing the first three games with the flu.
Despite allowing the first five shots of the game, the Senators opened the scoring on their first shot when Batherson beat Reimer midway through the first.
San Jose tied it early in the second period. Couture was left all alone at the corner of the crease and took a pass from Brent Burns before quickly going to his backhand and scoring into an open net.
Labanc made it 2-1 on a power play with 5:24 to go, beating Murray with a one-timer from the faceoff circle.
Senators forward Shane Pinto went down in the first period after taking a hard hit from Sharks defenseman Mario Ferraro. Pinto looked to be in significant pain as he left the ice with an apparent shoulder injury. He returned for a shift in the second period, but left again and did not reappear.
Sharks: At Toronto on Friday night.
Senators: Host New York Rangers on Saturday.