Kim Clijsters is doubtful for next week's Australian Open after a hip injury forced her out of the Sydney International on Wednesday.
"It's terrible," she told a news conference. "It's not good."
The Belgian, ranked second in the world, was due to play Italian Francesca Schiavone in the quarter-finals but withdrew after feeling a sharp pain during her warm-up.
Clijsters had problems with her hip last season and said she would not know the extent of her latest injury until she undergoes a scan on Thursday.
"I could hardly walk up the stairs. It feels like it's really deep inside," Clijsters said.
"I don't know what the injury is. I'll need to have some tests tomorrow."
Clijsters said she had minor back pain after flying in to Australia from Hong Kong where she played an exhibition tournament at the weekend.
"I don't know if that had something to do with it, maybe it was compensating for my back...I just don't know," she said.
"I hurt it warming up. I was hitting for about 20 minutes, just my ground strokes, and I was feeling fine.
"Then all of a sudden I just started to do serves and as soon as I served I felt some really sharp pains.
"I tried to stretch it out a little bit and do some movements to lossen up my hip or and my back but it just kept getting worse and worse."
Clijsters is one of the favourites to win the Australian Open, starting in Melbourne on Monday, after reaching the final two years ago and winning the U.S. Open last September.
The 22-year-old has been plagued with injuries since she sprained her ankle at the Hopman Cup in Australia two years ago.
She missed last year's Melbourne championship because of a wrist injury and said she could not bear to miss it again.
"That's something I don't want to think about yet," she said.