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.

“That’s probably the biggest part of it, to (sign) this extension and hopefully end my career here, that’s the biggest part of it,” Duncan said after the Spurs beat the Portland Trail Blazers 106-97 Tuesday night.

No official word has been released.

Spurs coach Gregg Popovich also spoke about the extension before the game.

“It shows him what we think of him and it adds continuity for all of the other players who are here — other players that we might want to sign in the future,” Popovich said. “It’s just part of doing the family thing and trying to make everybody feel as comfortable as we can within a business environment that we all have to live in.

“As many things as you can do to dilute the hard-core business end of it, you do it, and this is one of them if you can extend somebody.”

Asked about reports that Duncan took $10 million less than he was eligible to receive so the team would have salary-cap flexibility in the future, Popovich said Duncan is “definitely special and that’s just a very good example of how his brain works and what his priorities are.”

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.

Before beginning the defense of their title against the Trail Blazers, the Spurs were to be presented with 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.

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“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. 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.

Popovich also showed reporters gathered around him before Tuesday’s tip-off a cartoon he got from a newspaper several years ago. It depicted a player sitting in a main office, a coach waiting outside to see him and an administrative assistant. Popovich had labeled the player “TD” and the coach “Pop.” The cartoon’s caption read: “The Franchise will see you now, coach.”

“That’s actually how we work around here if anybody wants to know the truth,” Popovich said.