Dirk Nowitzki -- EPIC SHOWDOWN at Children's Hospital

Will Kolassa.

That name is going to haunt Dirk Nowitzki for years ... 'cause he DOMINATED the NBA superstar when a game of 1-on-1 broke out at a Children's Hospital in Dallas.

It all went down at Children's Medical Center in Dallas last week, where Dirk was handing out gifts to some of the kids.

And that's when it happened ... Dirk (7-footish) challenged Will (4-footish) to some mini-basketball ... and IT WAS ON!!!

Will set the pace early ... dropping a rainbow right over Dirk's head.

Sure, Dirk somehow managed to pull off a dunk ... and he chalked up a pretty nasty rejection ... but in the end, it was all Will.

The kid started off the day as a hardcore Bulls fan ... but now says he'll also root for Dirk.

We're guessing a lot of people are gonna feel that way after watching the clip.