In addition to faxing the official opt-out letter, Nowitzki had to mail a signed copy Monday from Germany to ensure its arrival at the Mavericks' offices before Wednesday's opt-out deadline in his current deal.
The move has been expected since an ESPN.com report in mid-May that Nowitzki planned to become an unrestricted free agent for the first time in his career. But Mavericks officials have remained confident that they will secure a verbal commitment from Nowitzki on a new four-year deal early in free agency.
Teams and free agents can begin negotiating on Thursday at 12:01 a.m. ET, but signed contracts can't be executed before July 8, when the league's moratorium on roster moves is lifted.
Mavericks president of basketball operations Donnie Nelson told The Associated Press on Tuesday that he has already booked a flight that will have him overseas when free agency begins at 12:01 a.m. ET on Thursday -- 6:01 a.m. in Germany -- to show Nowitzki how sincere the club is about keeping its all-time leading scorer and rebounder. But sources told ESPN.com that Nowitzki's schedule is still in flux and that it has not yet been determined whether he will be in his native country or in the U.S. when free agency starts.
"He is our No. 1 priority -- period," Nelson told the AP.
Nowitzki will forfeit next season's scheduled $21.5 million salary by opting out and is eligible to receive a four-year deal from the Mavericks worth a maximum of $96 million. Dallas is precluded from offering him a longer deal because of the league's over-36 rule, which prohibits giving players the league maximum if they will turn 36 before the contract expires. Nowitzki turned 32 earlier this month.
The Mavericks have spent the past several weeks trying to convince Nowitzki to sign an extension that would keep him off the open market. But opting out provides him with multiple contractual benefits. One of them is the ability to incorporate a no-trade clause into a new deal, which can be secured only by players -- in a new contract, not an extension -- with at least eight years of service time and four with the same team.
"It's a very odd, weird, strange time," Nelson said. "If there's anything good about this process, it's as someone goes through it, the thing that comes out is what is really important to them -- whether it's location, money, winning, those types of things. Those things really, really come out."
The Los Angeles Lakers' Kobe Bryant is the only player in the league with an active no-trade clause in his contract, but sources say a new Nowitzki deal with Dallas is expected to include a no-trade provision.
The Mavericks' dream scenario, according to sources with knowledge of the team's thinking, remains securing Nowitzki's verbal commitment as quickly as possible and then making him a central voice in their efforts to recruit the league's top free agents via sign-and-trade offers.
Sources have maintained since November that the Mavericks will make every attempt to convince LeBron James to push for a sign-and-trade to Dallas, despite Cleveland's insistence that it will not take part in a sign-and-trade and James' widely reported interest in possible moves to Miami or Chicago.
The Mavericks will enter free agency still believing they have an outside shot at landing James, although one source concedes that Dallas is no more than a "dark, dark horse" in the LeBron sweepstakes.
Sources say Dallas also intends to meet face-to-face with most of the elite free agents -- including Dallas native Chris Bosh, Joe Johnson and longtime Mavs nemesis Dwyane Wade -- in hopes of convincing another star to push for a sign-and-trade to play alongside Nowitzki and Mavs guard Jason Kidd, who at 37 remains one of the NBA's most popular figures among fellow players.
One source with knowledge of the team's thinking said Monday that the Mavericks "have an interest in any and all top free agents" for the "right sign-and-trade deal."
Sources said Sunday that the Mavericks are confident that they will be granted a turn to make their pitch directly to James in Ohio, thanks largely to the high regard members of James' inner circle have for Mavericks owner Mark Cuban and James' close relationship with Kidd.
As for Nowitzki, Kidd expressed confidence last week that the Mavericks can retain the leading scorer in franchise history, despite any interest he might generate on the open market.
"Sometimes as a player you want to feel wanted," Kidd said last week. "He's always felt wanted here, but maybe this could be some weird way of being appreciated by other teams. There's nothing wrong with that because he's probably one of the top five free agents or top four, top three. He's good.
"He'll get a lot of attention, but we all feel he's not going anywhere."
Cuban has made similar statements several times in recent weeks, despite the fact that the opt-out scenario does theoretically expose the Mavericks to the possibility of another team swooping in to sign Nowitzki away, as seen in the summer of 2004 when the Phoenix Suns stunningly pilfered Steve Nash.
Nowitzki has never been on the open market and only briefly flirted with a few American universities before it became clear that he would go straight from his hometown German club team to the NBA as a teenager.
"In some respects, it would confuse the marketplace, push things back for us," Nelson said of Nowitzki fielding offers from other teams. "But if there's a bug inside Dirk that wants to see what that's all about, then do it."
From the start, though, numerous executives around the league have expressed doubt that they can pry Nowitzki from Cuban's grasp, given Nowitzki's ties to Dallas after arriving with Nash on draft day in 1998 and the close bond Nowitzki and Cuban have forged through the owner's 10½-year reign.
Said Nelson last month, when the reality of Nowitzki's looming opt-out began to sink in: "That wouldn't change our approach one bit. We're going to do everything we possibly can to make sure Dirk is a Maverick for a long, long time. ... It's impossible for us to imagine Dirk in any other uniform.
"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."
Marc Stein is a senior NBA writer for ESPN.com. Information from The Associated Press was used in this report.