Williams had earlier in her career nearly 10 years ago battled some tendinitis in her knee before dealing with other injuries including a wrist injury which kept her off the tour for eight months two years ago.

Serena reached the semifinals last week at Stanford, steamrolling through what she called "an easy draw." Then in practice before her semifinal, she experienced some pain in her knee and was forced to retire from the tournament while trailing her opponent 2-6, 1-3.

This week Serena was scheduled to compete at Los Angeles at the East West Bank Classic with her first match tomorrow. But just this afternoon Williams announced her left knee had not yet recovered fully.

“I’ve been getting intensive therapy and doing everything in my power to get my knee in shape for this week, waiting until the last possible moment to see if I could play,” Williams said.

“Unfortunately after hitting this morning, I knew that I wasn’t going to be ready for this week. In the meantime, I’m working hard to be ready for the Olympics and U.S. Open.”

Both of the Williams sisters play light WTA Tour schedules even in good health and average only 13 or 14 tournaments per year. This accounts for lower than excepted WTA Tour rankings for both Venus and Serena as the WTA Tour takes a player's best 18 tournament results during the last 52 weeks and adds the rankings points together to obtain a player's ranking at the start of each week.

But eleven years after we first saw Venus Williams on tour fulltime, both Williams sisters remain in action, even while walking wounded, turning in top results as other former number ones and rivals such as Martina Hingis of Switzerland, and Kim Clijsters and Justine Henin both of Belgium have retired from the sport altogether.