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With NBA commissioner David Stern set to present Tim Duncan his fourth championship ring on Tuesday, the San Antonio Spurs forward has made it clear he has no plans to leave the court – or San Antonio – anytime soon.

Duncan has reached agreement with the Spurs on a two-year, $40 million extension that figures to give the team enough salary-cap flexibility to continue to surround him with a competitive support cast in the waning seasons of his career, two sources with knowledge of the deal said Monday.

By signing the extension, which is expected to be finalized within the next few days, Duncan forfeits his option to become a free agent after this season. He will be under contract with the Spurs until the summer of 2012.

The extension doesn't come as a surprise. Duncan, 31, said prior to last season's NBA Finals he felt as healthy as he has been in years and hoped to continue playing "as long as I can."

But what is notable about the deal are the terms: Although Duncan is eligible to receive a two-year extension worth about $51 million under the NBA's collective bargaining agreement, he agreed to nearly $11 million less because it could afford the Spurs greater flexibility to pursue free agents after the 2009-10 season.

Duncan and his agent, Lon Babby, met with Spurs coach Gregg Popovich and general manager R.C. Buford at Popovich's house prior to the start of training camp. Well aware of Duncan's value to the franchise – the Spurs likely would not be in San Antonio had he not guided them to their first title in 1999 – Popovich and Buford presented him with a maximum offer during the meeting. But they also made a detailed presentation of their plans for the team's long-term future and showed Duncan the possible impact of his accepting a lesser extension.

After a few weeks of deliberation, Duncan agreed to the $40 million extension. He will make $22.2 million in the final year of his current contract, with his salary dropping to about $18.7 million in the first year of the extension.

Duncan will be 34 when the extension begins. Tony Parker is the only other player currently scheduled to be under contract following the 2009-10 season, but the Spurs also will be in position to re-sign their third star, Manu Ginobili, that summer if they want.

The Spurs also are expected to eventually extend Popovich's contract to coincide with that of Duncan's.