Serena Williams might miss the rest of the 2010 season because her injured right foot hasn't healed properly.
"I am likely out for the year now," Williams said on Sunday in a posting on her official website.
Williams said she "re-strained" her foot by overtraining.
"In order not to make this worse, I have to chill on my physical workouts but I will immediately begin rehabbing as soon as I get the Doc's OK. Keep me in your prayers (I need them)," the posting said.
Williams' agent, Jill Smoller, confirmed the posting was from the 13-time Grand Slam champion and said she will be re-evaluated in the coming days.
If Williams is sidelined for the remainder of the season, it would be a blow to the WTA Championships later in October, as well as for the United States when it plays Italy in the Fed Cup final in November in San Diego.
Last week, Williams' older sister Venus said she will miss the rest of the 2010 season because of an injured left knee. Both Williams siblings had said in August they would make themselves available for the U S Fed Cup team in the final.
Serena Williams pulled out of the Generali Ladies Linz tournament that begins on Monday, saying she felt pain while training in Florida on Saturday, and a scan showed her injury hadn't healed properly.
She has been sidelined since July after cutting her foot on broken glass at a restaurant, shortly after winning Wimbledon. She had surgery that month.
She returned to practice in September but already put off her comeback by withdrawing from the Pan Pacific Open in Tokyo and the China Open in Beijing.
The reigning Australian Open and Wimbledon champion played only six tournaments in 2010 and will lose her No 1 ranking to Denmark's Caroline Wozniacki when the WTA releases the new list on Monday.
"I have been training for many weeks to get back to playing tennis as quickly possible since my foot surgery in mid-July," Serena Williams said on Sunday on her website.
"Yesterday I learned that I had tried too hard, and did too much too soon. After practicing yesterday morning I felt discomfort in my foot and tests by my doctor revealed that I had unfortunately re-strained it."