Duncan dominated inside with five blocked shots while Tony Parker helped out with 21 points, seven assists and three steals as the surging Spurs corralled their fifth straight win.
Tiago Splitter came off the bench to put in 14 points and gather six rebounds for San Antonio.
Memphis got 22 points and nine rebounds from Marc Gasol, but it wasn't enough to prevent a third consecutive setback.
Mike Conley provided 19 points, eight assists and six rebounds in the Grizzlies' loss.
Swim Week Miami Beach 2014 [PHOTOS]