Boston came into the contest having lost three of its previous four games, but Thomas backstopped the Bruins to a victory over Washington, turning aside 14-of-15 third-period shots while improving to 22-10-0 for the season.
Rich Peverley and Brad Marchand notched a goal and an assist each for the victors.
Marcus Johansson scored and Tomas Vokoun yielded three goals on 29 shots as the Capitals dropped their third in four contests.
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