Thomas played just a day after attending the funeral for his sister, who died in a car accident last month outside of Tacoma, Wash.
The Wizards were more energetic at the outset, jumping out to a 16-0 advantage and leading by as many as 17 points. But the Celtics rallied in the second quarter with Thomas briefly sidelined after getting one of his two front teeth knocked out.
A rivalry has been simmering between the teams since last year, when Boston swept the season series. They split their four regular-season matchups this season, but each included intense moments.
Sunday's matchup was played at just as high a level, with a frenzied pace at times and physical play underneath the basket.
After the Wizards' early scoring flurry, Thomas lost his tooth in a collision, and initially played through it. He found his tooth during a timeout, but started the second quarter on the bench while getting it repositioned in the locker room.
His teammates went on a 13-2 run without him to cut into Washington's lead.
Boston led by 15 at the end of the third before a 9-0 spurt by the Wizards to start the final period closed the gap.
Thomas arrived back in Boston early Sunday. It was the second cross-country trip for him since Chyna Thomas died in a car accident last month, the day before the Celtics opened their first-round series against Chicago.
The Celtics star flew with Boston president of basketball operations Danny Ainge and assistant coach Jerome Allen.