Nine-time Grand Slam champion Monica Seles has revealed she was diagnosed with myasthenia gravis – a neuromuscular autoimmune disease – three years ago.

The 51-year-old has chosen to go public with the rare long-term condition, which causes muscle weakness, to raise awareness before this month’s US Open.

Seles first noticed symptoms of the condition, which can affect most parts of the body – including the muscles that control the eyes, around five years ago.

“I would be playing [tennis] with some kids or family members, and I would miss a ball,” former world number one Seles told The Associated Press.

“I was like, ‘Yeah, I see two balls.’ These are obviously symptoms that you can’t ignore.

“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.”

Seles decided to reveal her condition in the hope of using her platform to educate people about the disease, for which there is currently no cure.

The American won eight major titles by the age of 19, after capturing her first aged 16 at the 1990 French Open.

But she won just one more after she was stabbed with a knife by a fan during a match in Hamburg in 1993 and took time away from the sport to recover.

Seles played her last match in 2003 having won 53 tournaments and spent 178 weeks at number one.