Cris Cyborg & Mia St. John Trade Shots in Boxing Sesh, Prepping for Holly Holm

UFC superstar Cris Cyborg’s got a secret weapon for her fight against Holly Holm -- boxing legend Mia St. John -- and TMZ Sports has footage of their intense sparring sesh.

Cyborg and MSJ traded shots for four 2-minute rounds at the RVCA gym in L.A. ... and you can tell they were taking it seriously -- even though Cris and Mia agreed not to go full beast mode before they stepped into the ring.

St. John actually faced Holly in a 2005 pro boxing match ... and even though Mia lost, we're told she was sharing tips on how to beat Holm with Cyborg.

After the gloves came off, MSJ told us -- "If Cris lands one solid punch, it's over for Holly."

If you're wondering why these 2 got together ... Cyborg's hitting up Mia's charity event on mental health awareness next weekend.

Pretty crazy to see these legends team up -- especially considering St. John recently called out Cris for a Playboy Mansion fight!