It's pretty amazing ... the 45-year-old ex-NBA superstar was playing some 5 on 5 (with a small goal) on Monday at an L.A. park with the likes of Team USA legend Cobi Jones!!!

Nash wasn't the most dominant guy on the field, but he certainly held his own ... which shouldn't be too big of a surprise considering he's a freak athlete who loves soccer.

In fact, Nash has a shrine in his L.A. home to some of his favorite soccer players ... so to get out there and ball out with Cobi Jones must've been awesome.

Of course, Jones is a Team USA soccer legend who played on the national team from 1992 to 2004 ... scoring 15 goals during his international career. He's the all-time leader in appearances for Team USA, with 164 caps.