Twice Wimbledon [ Images ] champion Serena Williams [ Images ] has withdrawn from this year's tournament because of injury, Wimbledon referee Andrew Jarrett said on Friday.

The 24-year-old American, who won Wimbledon in 2002 and again the following year, has been sidelined with a knee injury since she was knocked out in the third round of the Australian Open [ Images ].

Williams has played only four matches since last year's US Open [ Images ] and has been fined by the WTA Tour for pulling out of several tournaments this year.

She had knee surgery in 2003 and has not won a title since her seventh Grand Slam crown at the 2005 Australian Open.

"These past months have been very frustrating for me personally as I have had to take this time off to fully mend from a chronic knee injury," the former world number one said in a statement this month.