The Spurs stretched their winning streak to seven games, hitting 51 percent of their shots and making 9-of-17 from beyond the 3-point line.
Duncan had 28 points and 13 rebounds while Parker delivered 23 points and 11 assists. San Antonio led by as many as 24 points in the third quarter.
Brandon Jennings paced the Bucks with 31 points and John Henson had 20.
San Antonio has won six of its last seven meetings with Milwaukee. The Spurs will try to make it eight wins in a row Thursday in New York.