Former tennis superstar Monica Seles has revealed she has been diagnosed with a rare autoimmune condition, requiring a “hard rest” in her daily life.

The nine-time grand slam winner and former world No.1 said she first noticed the symptoms of the neuromuscular disease myasthenia gravis while she was swinging a racket.

“I would be playing with some kids or family members, and I would miss a ball. I was like, ‘Yeah, I see two balls’. These are obviously symptoms that you can’t ignore,” Seles said this week.

“For me, this is when this journey started. And it took me quite some time to really absorb it, speak openly about it, because it’s a difficult one. It affects my day-to-day life quite a lot.”

The 51-year-old Seles, who won her first major trophy at age 16 at the 1990 French Open and played her last match in 2003, said she was diagnosed with myasthenia gravis three years ago. She went public with it this week, ahead of the US Open, which starts on August 24, to raise awareness about what is known as MG.

The Australian government’s Health Direct website says myasthenia gravis is a “rare autoimmune disease affecting your neuromuscular system”.

“It causes a breakdown in communication between your nerves and muscles. As a result, the muscles that you voluntarily control become weak,” it says.

The condition is spread across all ethnic groups and is most common in females under 40 and males older than 60.

Symptoms of myasthenia gravis include:

• drooping of one of both of your eyelids
• blurred or double vision
• difficulty chewing or swallowing
• slurred speech
• weakness in arms and legs — including instability when walking
• chronic (ongoing) muscle fatigue
• shortness of breath

“Your symptoms can appear suddenly, or may take a few weeks to develop. Your muscle weakness can come and go,” Health Direct says.

“Myasthenia gravis symptoms get worse when you are doing activity. They get better after you rest.”

Seles, a member of the International Tennis Hall of Fame, said she’d never heard of the condition until seeing a doctor. She was referred to a neurologist after noticing symptoms such as double vision and weakness in her arms – “just blowing my hair out … became very difficult” – and legs.

“When I got diagnosed, I was like, ‘What?’,” said Seles, who is partnering with argenx, an immunology company headquartered in the Netherlands, to promote its Go for Greater campaign.

“So this is where – I can’t emphasise enough – I wish I had somebody like me speak up about it.”

Seles lives in the US and is married to American billionaire and philanthropist Tom Golisano.

It’s been three decades since she returned to competition at the 1995 US Open, making it to the final, more than two years after she was attacked by a man with a knife at a tournament in Hamburg, Germany.

“The way they welcomed me … after my stabbing, I will never forget,” Seles said about the fans in New York.

“Those are the moments that stay with you.”

She talks about learning to live a “new normal” since her diagnosis, and characterised her health as another in a series of life steps that required adapting.

“I had to, in tennis terms, I guess, reset – hard reset – a few times. I call my first hard reset when I came to the US as a young 13-year-old [from the then Yugoslavia]. Didn’t speak the language; left my family. It’s a very tough time,” Seles said.

“Then, obviously, becoming a great player, it’s a reset, too, because the fame, money, the attention, changes [everything], and it’s hard as a 16-year-old to deal with all that. Then obviously my stabbing – I had to do a huge reset.

“Then, really, being diagnosed with myasthenia gravis: Another reset. But one thing, as I tell kids that I mentor: ‘You’ve got to always adjust. That ball is bouncing, and you’ve just got to adjust’.

“That’s what I’m doing now.”

-with AAP