UFC's Daniel Cormier Sparring with Cain Velasquez ... 'Best Guy I've Ever Gone With'

Daniel Cormier is stepping up his training to prepare for Jon Jones -- sparring with 2-time UFC heavyweight champ Cain Velasquez.

Clearly, D.C. was impressed with Cain -- calling him "the greatest fighter I've ever been locked in a cage with."

"This is no joke @officialcainvelasquez is by far the best guy I've ever gone with."

Cormier vs. Jones is all set for UFC 200 on July 9th -- and D.C. is adamant he's finally going to get his revenge.