The duo was competing in the driveway basketball game at Team World practice. Rondae Hollis-Jefferson also took part in the showdown.
During Porzingis' turn to pick the shot, the New York Knicks sensation dribbled a few times before bouncing the ball off the backboard and throwing it down with his right hand.
Nowitzki drove to the rim and threw the ball off of the backboard, but instead of throwing it down, he was only able to tip the ball up in the air in failure.
"I wasn't loose yet," Nowitzski wrote on Twitter.
"Bring out the hops @swish41!! Matter fact save them when the cameras aren't around. #DirkWithTheBounce," James tweeted.
Nowitzki, 39, is set to return for his 20th season in 2017. The 13-time All-Star and 2006-2007 NBA MVP agreed to a two-year contract with the franchise this summer.
He is listed a 7-feet, 245 pounds while the 22-year-old Porzingis is 7-foot-3 and 240 pounds.
Team World beat Team Africa 108-97 Saturday in the 2017 NBA Africa Game.
Nowitzki had three points, three rebounds and two assists in the victory. Porzingis had 14 points, eight rebounds and three assists for Team World.