Ja Morant had a busy day in court Monday ... testifying that he punched a teen during an altercation at his home last year in self-defense -- while also demonstrating to an attorney how to check a ball in a pickup basketball game.
The Memphis Grizzlies superstar did it all before a judge in Tennessee ... as part of Joshua Holloway's civil case against him.
Holloway claims in the lawsuit that Morant assaulted him while the two were hoopin' at the NBA player's Shelby County residence in July 2022 .