Reigning Australian Open champion Maria Sharapova has withdrawn from this year's event because of a slower than expected recovery from a shoulder injury.
The Russian said she was making steady progress but was not fit enough to compete at the tournament, which starts on Jan. 19.
"I am very sorry to announce that I am not going to be able to defend my title at this year's Australian Open," the world number nine said in a statement.
"My shoulder is doing great but I just started training a few weeks ago and I am just not near the level I need to be to compete at the highest levels."
Sharapova won her first Australian Open title last year when she beat Serbia's Ana Ivanovic in the final at Melbourne Park but has not played since August.
The 21-year-old won her first 18 matches in 2008 to briefly reclaim the number one ranking before injury struck at Montreal, forcing her to withdraw from last year's Beijing Olympics and the U.S. Open.
"The Australian Open is one of my favorite tournaments, they have the best fans in the world and I am going to miss everything about the tournament," Sharapova said.
"I promise I will see everyone next year."
Australian Open officials, who announced on Saturday they were lifting prize money to record levels this year, said they were disappointed for Sharapova.
"I know she has been working around the clock to try and get back from the shoulder injury. In the end, time beat her this year," tournament Director Craig Tiley said.
"We wish her well with her return to tennis and look forward to seeing her back in Melbourne next year."
Sharapova is the first major casualty for this year's Australian Open but the fifth champion in the last eight years not to defend their singles title.
Andre Agassi (2001 winner), Serena Williams (2003), Justine Henin-Hardenne (2004) and Marat Safin (2005) also missed the first grand slam of the year because of injury.
Players have long been demanding the Australian Open, which is traditionally held in the last two weeks of January, be moved back to March to allow them to play in cooler conditions and enable them to take a longer break between seasons.
Tennis Australia officials did agree to push the 2007 tournament back one week but later scrapped the plan and reverted to the original dates.