It was the 40th straight 20-point game for Thomas, tying the team record set by John Havlicek during the 1971-72 season. Marcus Smart added 21 points and eight steals for the Celtics.
Boston has won four in a row and 11 of 12. Philadelphia's loss ended its three-game winning streak.
The Celtics won despite committing a season-high 24 turnovers, though they needed a late run to preserve the victory.
Dario Saric led the 76ers with 20 points and 11 rebounds. Robert Covington added 18 points, but left late in the fourth quarter after crashing into the Boston bench while diving for a loose ball.
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