There are few moments in sporting history that can be as accurately described as a sliding doors moment as that which happened to Monica Seles on this day 30 years ago.

By the age of just 19, Seles had established herself as the next superstar of the tennis world.

Born in the former Yugoslavia, she emerged as the only player capable of challenging Steffi Graf’s dominance. While still a teenager, she had already won eight Grand Slams.

It was clear she was a remarkable talent and with her double-handed shots on both forehand and backhand and her trademark grunt she definitely stood out from the pack.

But on April 30, 1993, Seles’ life changed forever. She was playing in an unremarkable match – a quarter-final against Magdalena Maleeva at a tournament in Hamburg. She was 4-3 up in the second set, having won the first, and appeared to be within minutes of winning the match.

But then one of the most shocking moments in sport occurred. An obsessed fan of Seles’ great rival, Graf, ran from the crowd on to the side of the court during a change of ends and stabbed Seles in the back.

“I remember sitting there, towelling 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 wrote in her autobiography.

“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.”

Seles was fortunate, if you can call it that. The incision was only an inch-and-a-half deep, and missed her spinal cord and other organs. Physically, the injuries were less severe than they might have been but the mental scars were catastrophic and took far, far longer to heal.

It was two-and-a-half years before Seles returned to tennis but she was never the same again.

In those few years before the attack, Seles had dominated Graf, who had previously been untouchable, having won eight of nine Grand Slams. Then, in the space of three years, Seles won eight major titles to Graf’s two and she was being talked about as potentially becoming the best ever.

But the stabbing changed everything, both for Seles and for sport, which was forced to face the reality that fans, whether mentally disturbed as in the case of Gunter Parche or otherwise, were not always well-meaning.

The most obvious change within tennis was that players began sitting side on to the crowd, rather than with their backs to them. But the long-lasting effects are debatable. It remains common to see fans invading the field of play in countless sports.

But it was the lasting consequences on Seles herself which were the most damaging. After being stabbed, the Yugoslavian-born American struggled with mental health issues as well as an eating disorder. Bouts of depression and anxiety led to binge eating and eventually weight gain.

Ultimately, Seles did make it back to the top, if only briefly. In 1996, a year after her return to tennis, she won a fourth Australian Open crown before adding Olympic bronze in 2000.

She played her last competitive match in 2003, a first-round loss at the French Open, but didn’t officially announce her retirement until 2008.

I’m usually not a fan of “what-ifs”. What-if this or what-if that is irrelevant in elite sport. All that matters is the final result. There is no room in this brutal, results-driven word for excuses or mitigating circumstances or claims of what could have been.

Except in the case of Seles, it’s impossible to avoid these what-ifs. What if she had not been attacked? She would likely have added substantially to her Grand Slam tally. She would almost certainly have ensured Graf never reached her final total of 22. The pair would, probably have created an extraordinary rivalry.

That all of this was never allowed to happen is a tragedy, more so when you take into account that the perpetrator, Parche, never even served any jail time.

These days, it is widely accepted that Serena Williams is the greatest ever female tennis player. With 23 Grand Slam titles, it’s hard to argue.

But it’s also hard to believe that she wouldn’t have had a serious run for her money for that accolade had Seles’ career not been so horrifically interrupted.

We all know how cruel sport can be. But there are few incidents to match that of 30 years ago, which irrevocably changed the course of Seles’ life.