The Dallas Mavericks are expecting star forward Dirk Nowitzki to opt out of his contract late next month and become an unrestricted free agent July 1, sources close to the situation told ESPN.com.

The Mavericks, though, continue to negotiate with Nowitzki on an extension in hopes of convincing him to preempt his free agency. They remain confident the leading scorer in franchise history will stay with the only NBA team with which he's played, even if Nowitzki winds up joining the most anticipated free-agent class in league history.

The sides have until the end of June to reach a deal to keep Nowitzki off the open market, but sources told ESPN.com this week that it's more likely -- with free agency less than 40 days away -- the nine-time All-Star becomes an unrestricted free agent for the first time in his career.

"That wouldn't change our approach one bit," Mavericks president of basketball operations Donnie Nelson told ESPN.com when asked about the prospect of Nowitzki exercising his opt-out clause.

"We're going to do everything we possibly can to make sure Dirk is a Maverick for a long, long time," Nelson added, describing team management as "hopeful, optimistic and expectant" about reaching terms with Nowitzki on a new deal.

Although the opt-out scenario would expose the Mavericks to the possibility of another team swooping in to sign him away, as seen in the summer of 2004 when the Phoenix Suns stunningly pilfered Steve Nash, numerous executives around the league remain doubtful that they can pry Nowitzki from the grasp of Mark Cuban, given the close bond they've forged through Cuban's 10½-year reign.

"It's impossible for us to imagine Dirk in any other uniform," said Nelson, who was instrumental in the pre-Cuban maneuverings on draft day in 1998 that landed Nowitzki and Nash in Dallas in separate trades.

"I think his heart is in Dallas. You learn to never say never in this business, but from our perspective we will be doing everything possible to keep him right here."

Nowitzki, who left this week for his annual return to Europe, could not immediately be reached.

Cuban likewise could not be reached but insisted in a recent radio interview with KTCK-AM in Dallas he believes Nowitzki isn't "going anywhere."

Even if his intention is to stay with the Mavericks as opposed to testing the market, Nowitzki has at least two contractual incentives to opt out and sign a new deal, as opposed to extending his current contract on top of his successful history with Cuban and the team's intention to flank him with a major sign-and-trade acquisition.

Opting out to sign a new deal, for starters, would lock in terms based on the NBA's current collective bargaining agreement through the life of the next contract. Under the league's current system, Nowitzki is eligible for a four-year maximum contract from Dallas worth $96.2 million once he opts out. The most he could receive from another team is a four-year deal worth $93.1 million.

Signing a three-year extension to the last remaining season on his current contract, by contrast, would expose Nowitzki to potential after-the-fact reductions to his annual wage if league owners are successful in their attempts to lower the value of maximum salaries in the next collective bargaining agreement.

Players with max-salary extensions that start in 2011, such as those signed earlier this season by Kobe Bryant and Pau Gasol of the Los Angeles Lakers, might be facing unforeseen rollbacks depending on how drastically salaries are reduced in a new CBA.

Dirk Nowitzki would join what is expected to be the best free-agent class in NBA history. D.Clarke Evans/Getty Images

Another motivation for Nowitzki to opt out as opposed to signing an extension is the ability to secure a no-trade clause.

Only players with at least eight years of NBA service time and four seasons with the same team are eligible to have a no-trade clause, but such clauses can only be added to new deals. NBA rules prevent major changes, such as adding a no-trade clause, to an existing contract or an extension, which is largely why Bryant possesses the league's only active no-trade clause.

Bryant secured a no-trade provision when he became an unrestricted free agent in the summer of 2004 and received a seven-year, $136 million deal from the Lakers. Yet most star players such as Nowitzki and Boston's Paul Pierce -- who will also have to decide by the end of June whether to opt out if he doesn't negotiate an extension -- sign extensions before they ever get to unrestricted free agency, denying them the opportunity to score a no-trade clause.

In a recent radio interview with KTCK-AM in Dallas, Cuban said: "Well, I can see the circumstances where he would opt out but not necessarily leave the franchise. ... Dirk told me that if he can help the team get better, he would sign a different deal. So we made the decision to see what was going to be out there and how things played out and work together."

By "different deal," it's believed that Cuban was suggesting Nowitzki will consider signing for less than a max contract if he's convinced that providing a discount will get him more roster help. The implication is that any financial relief through reduced luxury-tax payments Cuban gains in that scenario would encourage him to continue pursuing expensive trades, such as the pricy midseason swap with the Washington Wizards that landed Caron Butler and Brendan Haywood.

Another round of roster retooling is expected in Dallas because that trade -- although initially hailed as a potential landscape-changer in the West -- couldn't prevent the Mavericks from losing to longtime rival San Antonio in the first round, thus sparking fresh questions about the long-term potential of the Mavs' aging roster beyond Nowitzki and promising guard Rodrigue Beaubois.

Nowitzki turns 32 in June, which brings the league's over-36 rule into play. Because of that rule and Nowitzki's age, Dallas does not have the advantage of offering him a new contract that's one year longer than rival teams can offer. Cleveland's LeBron James and Miami's Dwyane Wade, for example, are young enough to command six-year deals from their current clubs as opposed to the maximum five years that can be offered by other teams.

The Mavericks also know that Nowitzki would almost certainly command a four-year max contract from teams such as the New York Knicks and New Jersey Nets if he fielded pitches on the open market, since he would instantly become one of the most coveted free agents in the 2010 class along with James, Wade, Chris Bosh and Amare Stoudemire. So Dallas must sell Nowitzki on its plan to stay in the West's elite to get him to consent to any sort of discount.

If the sides unexpectedly reach terms on an extension before free agency, Nowitzki is only eligible to receive an additional three years tacked onto next season's scheduled $21.5 million because of the over-36 rule.

"I don't think Dirk's going anywhere," Cuban said in his recent radio appearance. "I said the same thing about [Steve] Nash, but I don't think this is a similar situation.

"Dirk's been with this team more than 10 years versus four or five when Steve left, so it's a different situation. And I think Dirk is as committed as I am to bringing a championship to the Mavericks."

Re-signing Nowitzki is a must for the Mavericks on several levels. Not only have they won 50 games for 10 consecutive seasons with Nowitzki as their focal point, becoming just the fourth franchise in NBA history to do so. Nowitzki's presence is also central to Cuban's planned offseason pursuit of an elite free agent.

Sources say Dallas has been planning for months to use Erick Dampier's fully unguaranteed $13 million contract for next season and perhaps Butler or Haywood as trade chips in forthcoming sign-and-trade offers for free agents such as Cleveland's James and Atlanta's Joe Johnson, but attracting a player of that caliber rides on the prospect of teaming up with Nowitzki.

Although he has said many times winning a championship in another uniform wouldn't "feel right," given his long association with the Mavericks, Nowitzki would not commit to a return after Dallas' Game 6 loss to the Spurs. In an interview the day after the defeat, Nowitzki reiterated finishing his career in Dallas "was always my plan" but insisted that "I just have to keep my options open at this point" after the disappointment of the Mavs' third first-round exit in a span of four seasons.

The Mavericks privately acknowledge that Nowitzki, despite Dallas' habit of carrying one of the league's highest payrolls every season, needs more top-tier help to get the the team back to the Finals. Like James throughout his Cleveland career, Nowitzki hasn't played with a consistently All-Star caliber player since Nash's departure in 2004.

Even Nash's peak was reached in Phoenix in his back-to-back MVP seasons in 2005 and 2006, leaving Nowitzki to carry deep but otherwise starless rosters for the last six seasons.

Having seen the impact of Gasol's arrival in Los Angeles on Bryant's career and the difference playing with Pierce, Ray Allen and the emerging Rajon Rondo has made for Kevin Garnett, Cuban has spoken often of his determination to strike his own Gasol-style trade. Even if Beaubois keeps developing and ultimately lives up to optimistic comparisons with Rondo and Tony Parker, Dallas realizes it needs to flank Nowitzki with a elite sidekick.

"I don't think that you guys would disagree with me or that anybody listening would disagree that he's earned the right to take some time and think about things," Mavericks coach Rick Carlisle said during a recent "Galloway & Company" appearance on ESPN Radio in Dallas (103.3 FM).

"Let's face it: He has had a tough run here with playoff basketball and there have been some very disappointing endings to seasons. But I'll tell

you this -- and in the face of some of the personal things he's had to deal with -- this guy has been an absolute great player. Beyond belief. If you don't give him his due there, then you're not being right, you're not thinking straight and you're not telling it like it is.

"Give the guy some space. Let him think about what he needs to think about. He and Mark will get it worked out and things will go on from there. But he's earned the right to have some time to ponder things."

Marc Stein is a senior NBA writer for ESPN.com and a frequent contributor to ESPNDallas.com.