The team was playing a game of "rondo" and the Brazilian striker was one of the two players in the middle when, after the Uruguayan eluded him with a back-heel, he decided to give him a little nudge in the groin with his knee.
Suarez briefly pretended to be hurt, before laughing along with his team-mates.
Barcelona are favourites ahead of the second leg, having won the first match 3-1 in Paris.