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.

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. Before the game, they were presented their championship rings after sweeping LeBron James and Cleveland in June.

The Spurs are seeking their first repeat after failing to defend their titles in 2000, 2004 and 2006. They didn’t make it past the second round in any of those seasons.

“There’s no better feeling than being the last team standing,” Duncan said earlier this month. “And no matter how many times you attain that goal, you want to do it again.”

The quiet Duncan, who will be 36 at the end of the extension, is the foundation of the Spurs and their league-leading defense. Spurs coach Gregg Popovich’s hardworking and methodical approach are apparent in Duncan, who is known for almost unparalleled effort on and off the court.

Hobbled by plantar fasciitis in previous seasons, Duncan was healthy last year and has said he feels ready again this season.