Monica Seles has revealed that she was diagnosed with myasthenia gravis, a neuromuscular autoimmune disease, three years ago.

The nine-time Grand Slam champion has opted to go public ahead of the US Open in a bid to raise awareness of the condition.

She first noticed symptoms of the muscle-weakening disease while playing tennis around five years ago.

"I would be playing with some kids or family members and I would miss a ball," 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."