Four-times champion Andre Agassi could miss next week's Australian Open after he pulled out of the Kooyong Classic with a hip injury on Thursday.
The 34-year-old was trailing fellow American Andy Roddick 6-5 when he retired.
Agassi is due to have an MRI scan to reveal the extent of the injury but he said it was not the same problem that caused him to miss several tournaments last year, including Wimbledon.
"This, I felt it as a muscle, the flexor," Agassi told reporters. "I felt it a little tight, then I felt a sharp pain.
"I didn't know if it was one of those quick feelings you get if you don't stretch right to a ball and then it goes away. But then when I sat down on the changeover and I stood up to go, I had a hard time lifting my leg to stand up."
IMMEDIATE TREATMENT
Agassi apologised to the Kooyong crowd for pulling out, explaining that he needed immediate treatment. He said it was too early to say whether he would be fit for the year's first Grand Slam event, which starts on Monday.
"That's guesswork," he said. "I'll have to wait and see what I am dealing with, to be honest.
"The good news is that it didn't sort of just tear. It was tightening up, so that tends to be your body protecting itself, which hopefully was more the issue."
Should Agassi be forced out of the Australian Open, he would join a mounting casualty list.
Australian Mark Philippoussis confirmed his withdrawal on Thursday due to the adductor muscle tear he suffered at the Hopman Cup in Perth last week.
Former world number two Tommy Haas of Germany is another doubtful starter with a similar injury while Swede Joachim Johansson faces a race against time after hurting his hamstring on Wednesday.
The women's event has already lost defending champion Justine Henin-Hardenne, last year's runner-up Kim Clijsters and twice champion Jennifer Capriati, all to injury.
Agassi won the Australian Open in 1995, 2000, 2001 and 2003 and is among the favourites again this year.
