Different sports. Similar dynasties.
"They're built and designed culturally very similarly," said Scott Pioli, head of the Patriots player personnel department during the championship seasons, "knowing who the leader is and making sure that people are going to fit in with what the leadership wants and how they want to do it."
Stability at the top is crucial to both teams' success.
Brady is in his 15th season, Duncan his 18th. They're humble superstars who set a team-first example.
Bill Belichick has coached the Patriots to 11 division titles in the last 12 seasons and has home-field advantage throughout the AFC playoffs for the fourth time in five years. Gregg Popovich has led the Spurs to the playoffs in 17 of his 18 full seasons as coach.
Longtime owners Robert Kraft and Peter Holt have made it clear who has authority -- Belichick on the Patriots, Popovich and general manager R.C. Buford on the Spurs.
And there's mutual admiration.
"Any successful franchise has good synergy between ownership, management and coaches," Popovich said. The Patriots "obviously have that. It's a fantastic organization."
Belichick returned the compliment.
"I love the way he coaches that team," the Patriots leader said. "I admire it."
Pioli is very familiar with both clubs.
He was with the Patriots from 2000-08 and has had many discussions about team building with Buford, a close friend.
"Having a locker room that is dominated by the right kind of people and the right makeup is just as important as talent if you want to sustain championship level football or basketball," said Pioli, now assistant general manager of the Atlanta Falcons. "The key (was) to make sure that we went out and got players that loved and were willing to commit to football the way that the head coach would. And that's what San Antonio does."
Belichick and Popovich are the longest serving active coaches in their leagues. They demand precision in practices and games, stress team over individual success and treat their stars like their other players.
When they mess up, Brady and Duncan hear about it.
"The Spurs and the Patriots have a collection of guys that are over themselves," said Los Angeles Clippers coach Doc Rivers, who became friendly with Belichick while coaching the Boston Celtics. "Pop talks about that all the time, and so does Belichick: `I need guys to be over themselves and about the team."
"When you have a superstar player that is humble and buys in and is willing to do what's best for the team, that sets the tone for the rest of the team," Patriots special teams star Matthew Slater said. "Looking at Tim Duncan from afar, you definitely see that. And then being in the locker room with Thomas, it's the same thing."
That constant excellence has come in an era when parity and salary caps mitigate against it.
The Patriots won Super Bowls in the 2001, 2003 and 2004 seasons. Since Brady became a starter in 2001, they have the best record in the four major American pro sports leagues with the Spurs second.
The Spurs won NBA titles in 1999, 2003, 2005, 2007 and 2014. Since Duncan arrived in 1997, they're first in winning percentage with the Patriots second.
"Your success is very much dependent on the person next to you and their ability and dependability in those two sports," Pioli said. "It's about the players being able to respond to the great leader."
Some luck helped.
Brady was still available in the sixth round when the Patriots drafted him in 2000. Duncan was a cinch to be the No. 1 pick in 1997 and the Spurs got it even though the Celtics had the best chance to win the lottery.
Both organizations have found veterans who could contribute.
Linebackers Mike Vrabel and Larry Izzo joined the Patriots in 2001 and played key roles in all three championships. Running back LeGarrette Blount and linebacker Akeem Ayers have played well since their midseason additions this year.
"Bill needed smart players that were tough and low maintenance," Pioli said.
There are, of course, differences in how the franchises have thrived.
"The longer you spend in this industry, you learn that there is not one formula for success in professional sports," Kraft said. "What works for one organization doesn't necessarily translate to another. But what is important is that you find a formula that works best for you. Clearly, the Spurs have found a formula that works best for them."
Just because Patriots assistant coaches learned from the master, success didn't follow them -- Romeo Crennel, Eric Mangini, Josh McDaniels -- when they became NFL head coaches.
So why does New England keep winning?
"That comes down to three people," Slater said. "Mr. Kraft, Coach Belichick and Tom Brady."
The Spurs have a similar foundation.
"What is the goal? Do you want to win just one championship or do you want to sustain it?" Pioli said. "When your core chemistry is hitting on all cylinders, that's when you have a chance to win championships."