The Trail Blazers scored the first eight points of the game and never trailed, though the Spurs got to within five points late in the game before Travis Outlaw scored on a jumper and Steve Blake hit four free throws in the final 30 seconds.
The Spurs were led by Richard Jefferson's 19 points and Manu Ginobili's 17. Duncan had 14.
San Antonio's Tony Parker sat out the second half with an injured ankle.
The Blazers shot 90 percent from the free throw line and scored 12 fast-break points to San Antonio's two.