There is no single reason Tim Duncan is still playing All-NBA level basketball just days before his 39th birthday. (Fun fact: Duncan enrolled at Wake Forest a month before his teammate Kyle Anderson was born.)

His work ethic has long been lauded. He keeps himself in impeccable shape. He's possibly the most coachable superstar in NBA history, which helps him fully accept Gregg Popovich's lessons. He's one of the smartest players in the NBA. He's had great teammates throughout his career, which allows him to carry a smaller load than his age would suggest. He's avoided the added wear and tear from international competitions since 2004.

But most of all, Duncan has always had a type of game that ages well because it's based thoroughly on skill. They call him Big Fundamental for a reason.

Duncan was actually sneaky athletic early in his career. Until recently, you could even call him agile. Those attributes were most often seen on defense, where Duncan covered the pick and roll as well as any big man in the game and defended the post beautifully. On offense, Duncan has always been a master of executing the pick and roll -- especially since Tony Parker developed into a star point guard in the mid-Aughts -- and is a scary face-up player. He's been trotting out that fabulous elbow bank shot for decades. How many defenders have watched Big Fun hit one in their face? A hundred? 200?

On Wednesday, Duncan racked up 28 points and 11 rebounds to lead the Spurs to a Game 2 victory. Most of those points came against DeAndre Jordan, who finished third in Defensive Player of the Year voting. Jordan is exactly the type of player Duncan never was: A raw athletic powerhouse without touch. Jordan is exactly the type of center most teams want these days, in large part because of plays like this one from the fourth quarter of Game 2.

The NBA is a league of athletes. Center prospects are graded first by length and athleticism and only after that by skill level and basketball IQ. (Basketball IQ is an awkward term, but it's also well-known and fairly understood.) Centers like Jordan are very much in vogue, and plays like the one above are a huge reason why. You can't teach 7 feet, they say. You also can't teach 7'6 wingspan or pogo-stick legs.

Jordan has learned to read the floor over the years, and while he can't dribble, shoot or pass with much success, he has picked up some key basketball skills. You can teach a prospect like Jordan how to do those things if he's willing to learn. You can't teach a prospect like, say, Frank Kaminsky how to jump over a guard to finish an alley-oop. If you believe in the growth mindset and a player's willingness to learn, you'll pick the more athletic prospect every time.

Duncan came into the league as a skill guy, but as a skill guy so dominant, smart and reliable that he was a surefire star. That was a different time. If he entered the league now, he might be edged out for No. 1 overall by a more springy, explosive big man or a lightning-bolt wing. But the fact that his top advantage has always been his advanced, unrivaled skill level has allowed him to survive a loss of whatever athleticism and agility he brought to the league.

He has clearly aged, despite keeping himself in excellent shape. You can see it on defense most often, especially when Duncan is trying to mark a young bull like Blake Griffin. You can see it in that rejected turnaround attempt on Jordan. Those summer sprint workouts and all that in-season rest helped preserve what physical advantage remains. Yet without all that fundamental skill -- the face-up moves and shots, the perfect footwork, the court-reading ability, the learned-by-rote-and-study tips and tricks of rebounding, defending and scoring in traffic -- Duncan would be a total shell.

Can you see DeAndre Jordan remaining effective into his late 30s when his legs stop launching him two feet off of the ground to finish dunks and he's not quick enough to rotate for a monster block? This isn't meant to be a criticism of Jordan. Very few people can age like Tim Duncan.

This brings us to Griffin, who has spent this season showing us how well-rounded he's become. In his early years, Griffin was exactly what the NBA loves right now: An athletic monster who attacked with abandon and dunked everything he could. He actually had some nascent skill even back then; he's always been more versatile than critics say. Griffin was never a smaller DeAndre Jordan.

But in the last couple seasons, he's taken all of that to another level. That was on display on Wednesday as he picked up his first career playoff triple-double.

There's been some noise that Griffin should have kept his old offensive style because that Blake Griffin was more difficult to defend. The lesson of Duncan's longevity, though, suggests that so long as Griffin remains in great shape and continues to work hard for the next decade, he can stay alive in the NBA long after his fast-twitch fibers fade.

Springs for legs make you millions in your 20s. Big Fundamental skills win you rings in your 30s.

SB Nation presents: Watch Blake Griffin posterize the defending champs