Dating NFL QB Mason Rudolph!!!

Call this a new kind of love-love for tennis star Genie Bouchard ... TMZ Sports has confirmed she's now dating Pittsburgh Steelers QB Mason Rudolph!!!

A source close to the couple tells us the two began a romantic fling recently ... and have gotten pretty serious.

In fact, 26-year-old Genie was just spotted at a fancy restaurant in Pittsburgh this week ... posting a pic of her at the place with the caption, "Pitt stop."

25-year-old Mason -- who was presumably on the other end of the camera -- dropped a "like" on the photo.

Of course, the new relationship means the end of a possible old one for Rudolph ... as we previously reported, the quarterback had been seen in L.A. in April getting cozy…