Tim Duncan's trainer says he could compete with MMA fighters

San Antonio Spurs legend Tim Duncan is keeping himself in top shape in retirement, unlike longtime nemesis Kobe Bryant.

In addition to frequenting the Spurs' practice facilities, Duncan is also training in mixed martial arts to whittle away his well-deserved retirement after nearly two decades in the NBA.

Here's some footage of Duncan's MMA workouts posted by his trainer Jason Echols:

Echols told Joon Lee of Bleacher Report that training Duncan has been a difficult challenge, mostly due to the size mismatch. Echols tore his bicep trying to keep up with Duncan during one session, and bragged that his client could compete with MMA fighters.

"(Duncan) would be a legit competitor, for sure," Echols said. "He has a desire to learn and compete. He has the heart for it. He most definitely would be a competitor.

"I would highly encourage him not to. Being a retired Spurs player, I wouldn't want him going out there and getting punched and kicked by some of those guys, but Tim would be a competitor. David would learn to become one. They are both competitive guys and not used to losing."

Joining Duncan in training is fellow Spurs champion David Robinson, who is a decade older and newer to the sport. Echols said the two Spurs have sparred and teased that some footage might eventually emerge.

"Maybe the public will see (that) in the coming days," Echols said. "You never know."