Pras Michel has his game face on as his federal conspiracy case kicks into high gear in Washington D.C., with some huge names -- including Leonardo DiCaprio -- set to take the stand.

We caught The Fugees rapper just as he arrived at the courthouse with his defense lawyer ... ahead of what could be a crucial day. Leo is a key witness, and he testified Monday morning after the court allowed him to use a side entrance to avoid the media.

TMZ Hip Hop broke the story back in October ... Leo is just one of the many high-profile witnesses who could take the stand before it's all said and done.