Kevin Garnett added 20 points and the Celtics, winning for just the second time in five games, moved within two games of Chicago for the No. 1 playoff seed in the East.
The Spurs stayed stuck in an untimely tailspin. The NBA's winningest team lost its fifth in a row and is in danger of falling out of first place after comfortably being No. 1 in the West all season.
Tony Parker led the Spurs with 23 points and Tim Duncan, playing for the first time in five games after spraining his ankle last week, had 20 points and 13 rebounds.