Former world number one Serena Williams, a seven-time Grand Slam champion, said Wednesday she will miss Wimbledon and the French Open this year because of a chronic left knee injury.

The 24-year-old American is only 21 this season, reaching the third round of the Australian Open in a title defence in her only appearance so far this year.

"These past few 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," Williams said.

"What I can tell you today based on what I am being told by my doctors is that I should be in a position to be playing again by the end of this summer."

That could put her in position to play in the US Open, where she won her first major title in 1999.

Williams took nine months off in July of 2003 after winning Wimbledon. She returned in March of 2004 and won a tournament in Miami.

Williams won four Grand Slam titles in a row starting at the 2002 French Open and also captured Wimbledon in 2003 as well as the 1999 US Open and 2005 Australian Open.

The announcement came on the same day that her sister Venus Williams, the reigning Wimbledon champion, won her first match of 2006 at the Warsaw Open. She had played only two matches since a US Open quarterfinal loss last year.