San Antonio led by 23 at halftime and increased the advantage to a massive 39 points heading into the fourth quarter.
Duncan led the way with 21 points and 16 rebounds while playing just 24 minutes. Tony Parker added 15 points and Kawhi Leonard had 13 as six San Antonio players reached double figures.
It was San Antonio's ninth win in 10 games against Eastern Conference opponents. The Bucks have lost four out of six and have just five wins for the entire season.
Milwaukee came up with almost half its points from two players -- Nate Wolters with 18 and Giannis Antetokounmpo with 15. Both are rookies and both had a career high in points.