Justine Henin-Hardenne has pulled out of this month's Australian Open after injuring her right knee.
"It is disappointing because I was so happy to be back and playing," said the 2004 champion, who could be out for between four and six weeks.
"The rest of me is feeling good after last year which was so difficult. I have to be careful and smart but it is hard to accept this decision."
The Belgian played only nine events last year due to a mystery virus.
Her last tournament was in September, when she lost in the fourth round of the US Open.
Henin-Hardenne has a "micro fracture" of the femural condyle bone but does not know what caused it, although she said it happened about 10 days while training at her Florida base.
Fellow Belgian Kim Clijsters, who lost to Henin-Hardenne in the final last year at Melbourne Park, has also confirmed she would not be competing in the first Grand Slam of the year.
She has withdrawn because of a persistent wrist injury.
Clijsters had surgery on her problem left wrist in May and made a comeback in September, but she has now decided to delay a return until mid-February.
"Kim doesn't feel like putting too much stress on the joint that early and wants to avoid any possible risk," a statement on her website said.
Provided she does not aggravate the injury in the meantime, Clijsters intends to make her comeback at the Antwerp Proximus Diamond Games on 14 February.
But, should the 21-year-old feel ready a week earlier, she might play in the Open Gaz de France tournament in Paris - as defending champion she can count on a wild card there.
Clijsters is still ranked 22nd in the world despite only playing a handful of matches last season.