SAN ANTONIO - Tim Duncan has agreed to a two-year, $40 million contract extension with the San Antonio Spurs, an official familiar with the negotiations said Tuesday.

San Antonio Spurs forward Tim Duncan (21) blocks a shot by Orlando Magic forward Rashard Lewis (9), during the first half of an NBA preseason basketball game in Orlando, Fla., Thursday, Oct. 25, 2007. (AP Photo/John Raoux)

The agreement will add to Duncan's existing three-year deal and keep him in San Antonio at least through the 2011-12 season, according to the official, who spoke on condition of anonymity because the source was not authorized to discuss negotiations.

The 31-year-old Duncan has led the Spurs to four NBA titles in the past nine seasons, and he was the finals MVP for the first three. He also won back-to-back league MVP titles in 2002-03.

Duncan, entering his 11th season, was the No. 1 pick in 1997, two years before the Spurs won their first title.

The nine-time All-Star has career averages of 21.8 points and 11.9 rebounds.

The Spurs opened defense of their title Tuesday night at home against Portland.