Tennis Player Nick Kyrgios -- Ultimate Trash Talk in Match ... Your GF's Cheating, Mate!

Even John McEnroe might say this crossed the line -- tennis player Nick Kyrgios telling his opponent, DURING a match ... that his girlfriend is juggling another tennis player's balls.

The supreme trash talking went down during a televised match in Montreal ... Kyrgios had just aced French Open champ Stan Wawrinka, and as he prepped for his next serve, Kyrgios mutters, "Kokkinakis banged your girlfriend. Sorry to tell you that, mate."

Unclear if Wawrinka heard him from cross-court, but spectators and the TV audience definitely did. More importantly ... we have no idea if Australian Thanasi Kokkinakis really did have sex with Wawrinka's chick ... tennis player Donna Vekic.

Point is ... Kyrgios is ready for the NBA with a mouth like that.

Wawrinka is demanding the ATP punish Kyrgios.