On April 30, 1993, then-world No. 1 Monica Seles was playing Magdalena Maleeva in the Citizen Cup, an undistinguished event in Germany. Seles was up 4-3 in the second set after having won the first, and appeared to be within minutes of taking the match and moving onward.

And then, during the changeover, the entire world of tennis changed in an instant. Gunter Parche, an unemployed 38-year-old, leaned over and stabbed at Seles with a nine-inch blade. Parche later admitted an obsession with Steffi Graf, Seles' rival, and sought to end the rivalry himself.

"I remember sitting there, toweling off, and then I leaned forward to take a sip of water, our time was almost up and my mouth was dry. The cup had barely touched my lips when I felt a horrible pain in my back," Seles would later write in her 2009 autobiography "Getting a Grip." "My head whipped around towards where it hurt and I saw a man wearing a baseball cap, a sneer across his face. His arms were raised above his head and his hands were clutching a long knife. He started to lunge at me again. I didn't understand what was happening."

Here is video of the aftermath:

Security and spectators restrained Parche before he could stab again. Seles was fortunate; the incision was only about an inch and a half deep, and Parche just missed her spinal cord or other organs. Seles was only slightly injured physically, but one could argue that the psychological scars never healed.

It's impossible to overstate how the Seles stabbing affected not just her career, but the entire game of tennis and, to some extent, pro sports as a whole. Seles was, in both ranking and demeanor, atop the world. At age 19, she was coming off a run of 22 straight singles titles, and held eight Grand Slam tournaments. She and Graf appeared poised to give tennis fans a rivalry to echo Evert-Navratilova and Sampras-Agassi.

But in the wake of the incident, Seles would stay away from the game for more than two years. While Seles would win one more major, as well as an Olympic bronze medal in 2000, Graf, without a true rival, completely destroyed the women's game, winning a total of 22 majors in her career.

Astonishingly, Parche received only a suspended sentence and probation, as the German court charged with rendering judgment dismissed an attempted murder charge. A later court upheld the verdict, in part because Seles did not want to come back to Hamburg and sit in a courtroom with Parche.

"I think that [got to me] more than anything, that there was no kind of punishment," Martina Navratilova told ESPN. "The judge was like, 'Oh, he won't do it again so I'll let him go so he can really kill someone.' It was insane and so nationality driven. If someone had done that to Steffi so Monica would win, they'd have thrown away the key."

Seles would eventually retire in 2008, although she hadn't played her last competitive match in five years. She would be elected to tennis' Hall of Fame the next year.

Fortunately, there have been few cases of fans getting onto the field and directly assaulting athletes since then. Perhaps the most notable was an incident in 2002 when a father and a son ran onto the field during a White Sox game and tackled Kansas City Royals coach Tom Gamboa; a pocketknife was found at the scene of the incident. But the players' vulnerability remains.

And even in tennis, the players are not completely safe. A fan charged Roger Federer at the 2009 French Open, prompting NBC's Ted Robinson to remark, "there should be zero tolerance for that, and of all sports, this one is the one that experienced the absolute worst with the nightmare of Monica Seles."

Seles remains in the public eye, having appeared on "Dancing With the Stars" and penned both an autobiography and an upcoming children's book. Still, her every move is haunted by what might have been.

"She was dominating Steffi Graf, who, prior to Seles, dominated everyone else," Pam Shriver told ESPN. "The sad thing about the whole thing to me was that besides the physical and emotional harm that was done to Monica, one of our great champions, is that this guy, in the end, got exactly what he wanted."