Duncan, held out of Tuesday's startling 107-83 loss at Northwest Division bottom-feeder Minnesota with a sore left knee, racked up 28 points and 19 rebounds to lead the way as the Western Conference front-running Spurs secured a postseason berth for a 16th consecutive season.
Gary Neal added 16 points and Kawhi Leonard, also rested during Tuesday's setback, had 12 along with six rebounds for San Antonio.
The Mavericks entered the contest having matched a season high with four straight wins, and nearly made it five after scoring seven unanswered points to pull within 92-91 in the closing seconds. Dallas got the ball back with a chance to prevail, but Vince Carter's 3-point try rimmed out at the buzzer.
Dirk Nowitzki finished with 21 points and 11 rebounds for the Mavs, with Darren Collison netting 12 points in the setback.
San Antonio hosts Cleveland on Saturday. Broadcast time is 7pm on 1200 WOAI.