Marat Safin is increasingly unlikely to defend his Australian Open title because of a knee injury, tournament director Paul McNamee said on Tuesday.
Safin has not played since August and is facing a race against time to be ready for the Australian Open, starting at Melbourne Park on January 16.
The Russian announced his withdrawal from the Hopman Cup, one of the lead-up events for the Open, in a statement on his website.
"Not playing this tournament, which was an important part of his preparation for Melbourne, certainly puts him behind the eight ball," McNamee told reporters on Tuesday.
"There is certainly a big question mark over his participation in the Open."
Safin won his first Australian Open this year, beating Roger Federer in the semi-finals then coming from a set down to defeat Lleyton Hewitt in the final.
He has been plagued by knee trouble since tearing a ligament at Wimbledon and has not played since the Cincinnati Masters Series in August.
McNamee said he is still waiting on news about Spain's French Open champion Rafael Nadal after the world number two withdrew from the Indian Open in Chennai.
Nadal told organisers he was pulling out of next week's season-opening event because of a foot injury that forced him to miss the Masters Cup.
"There is no more information other than he's pulled out of Chennai," McNamee told Australian radio.
"Fingers crossed there. He's obviously the number two for Melbourne and a pretty big guy there."
Australian Open organisers are also hoping Maria Sharapova and four-times champion Andre Agassi will recover for the event.
Sharapova withdrew from the Australian women's hardcourt championships, which she had intended to use as a warm-up, because of a shoulder problem while Agassi is battling a long-term ankle injury.
Players have been asking for the first Grand Slam of the year, which is traditionally held in the last two weeks of January, to be moved back to March when the weather is cooler and also to give them a longer break.
It is not unusual for top players to miss the Australian Open through injury and this year's casualties included defending champion Justine Henin-Hardenne, beaten finalist Kim Clijsters and Jennifer Capriati, the 2001 and 2002 winner.
Tennis Australia officials did agree to push the 2007 tournament back one week but later scrapped the plan and reverted back to the original dates.
