You know it’s a pretty impassioned rivalry when an obsessed fan goes and stabs the competition. In the early 90s, Steffi Graf and Monica Seles were two of tennis’ most powerful and unbeatable female hitters — from 1991 to 1993, not a single Grand Slam tournament was won by a female that didn’t have the last name Seles or Graf. During the late 1980s, before Seles showed up, Graf was undeniably the best female player in the world, but when the 16-year old, armed with a commanding two-handed backhand and an oddly distracting grunt, defeated Graf in the finals of the 1990 French Open (becoming the tournament’s youngest champion), it was clear that there was a new heir to the throne. The two players went on to meet in a total of six Grand Slam finals — each taking three — before Seles was shockingly (and literally) stabbed in the back by Gunter Parche, an obsessed Graf fan, at a match in Hamburg in 1993, ending what promised to be one of most intense and enduring rivalries in tennis.