Free agent guard Jason Terry is closing in on a three-year contract with the Boston Celtics, sources close to the situation confirmed Tuesday.

But one source close to the situation stressed that Terry will give the Dallas Mavericks a chance to counter before finalizing the pact believed to be for the full mid-level exception of $5 million annually.

Terry has spent the past eight seasons in Dallas, having replaced Steve Nash in 2004. He won the Sixth Man of the Year Award in 2009 and had a phenomenal playoff run in 2011 to help the Mavs win the franchise's first NBA championship.

Dallas, which is still waiting for a decision from Deron Williams of whether he will re-sign with the Brooklyn Nets or join his hometown Mavs, likely will be unable to sign Terry if Williams comes aboard.

A second source said that Terry's decision to move forward on a deal with Boston was made with no knowledge of Williams' plans to either stay in Brooklyn or join the Mavs.

Terry, who will turn 35 before the start of next season, averaged 15.1 points and 3.6 assists last season and shot 37.8 percent from the 3-point range.

Fourth on the NBA's all-time list for 3-pointers made, Terry is a 38-percent 3-point shooter for his career and has averaged 16.1 points per game.

For the latest, hop HERE.