Graf was not in her prime when she began losing to Seles. She was already dealing with nagging injuries to her back and hips, and unlike Seles who only had to worry about her tennis up until the tragic incident, Graf had to deal with litigations, her father going to jail, family falling apart, etc.
Mental strength is a strange thing. Something snaps and you can lose all of it rather suddenly. For example, Mats Wilander, who had one of the toughest will to win, achieves the unthinkable, beating Lendl by attacking the net, and then at the height of his achievement, he quits. He later described it this way: he knew he had given everything he had. He had a good idea about his tennis potential, and he knew that he had exhausted every aspect of it. So there was nothing more for him to achieve in tennis.
When Seles was stabbed, I bet it changed her perspective on life. She was no longer a teenager whose mind was set only on winning tennis matches. She probably did a lot of soul searching, gaining a broad perspective on life. But that seems to have taken away the mental edge that served her so well in tennis prior to the incident.
Over all, Graf was a much better athlete. An uninjured Seles would have soon found touch challenges from another mental giant Hingis, followed by power houses such as the Williams sisters.
