Couture broke a scoreless tie with 8 minutes left in the game, scoring from a sharp angle while skating down the right side, and 3 minutes later Boyle made it 2-0 on a one-timer off a Dany Heatley feed from behind the net.
Niemi turned aside all 25 shots he saw, including 12 in the third period, notching his fourth shutout of the year as San Jose improved to 8-0-1 in its last nine games and pulled to within one point of Dallas atop the Pacific Division.
Michal Neuvirth stopped 34-of-36 shots for the Capitals, who remained three points behind Tampa Bay in the Southeast Division standings.
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