Twice Wimbledon champion Serena Williams has withdrawn from this year's tournament because of injury, Wimbledon referee Andrew Jarrett said on Friday.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

Williams has played only four matches since last year's U.S. Open 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.