A dejected Serena Williams has no idea when she will be able to return to action after her hopes of a glorious grand-slam comeback were dashed by a pectoral injury.

Thousands who flocked to Roland Garros on Monday were left stunned when Williams announced that she had withdrawn from the French Open just before she was due to face old foe Maria Sharapova in the fourth round.

The 23-time grand slam singles champion had showed great promise in her first major since giving birth to daughter Alexis Olympia last September.

Williams first felt the injury, which she has never suffered before, during her third-round victory over Julia Goerges on Saturday.

The 36-year-old revealed the painful issue has left her unable to serve and she had played a doubles match with sister Venus on Sunday to see if taping and strappings would ease the pain.

Williams, who spoke softly and was understandably despondent when she faced the media, will have a scan in Paris on Tuesday to discover the extent of the damage.

"I'm going to get an MRI tomorrow. I'm going to stay here and see some of the doctors here, see as many specialists as I can. And I won't know that until I get those results."

She added: "It's really difficult to be in this situation, but I always, for now in my life, I just always try to think positive and just think of the bigger picture and hopefully the next events and the rest of the year."

Despite being uncertain about the severity of the setback, Williams is optimistic that she will return in the near future.

The American stated: "I can only take solace in the fact I'm going to continue to get better. And I had such a wonderful performance in my first grand slam back.

"I just feel like it's only going to do better and I'm coming up on hopefully surfaces that are my absolute favourite to play on and that I do best on.

"Hopefully I can continue to heal and be able to play those events."