Keothavong posted she had 'busted my knee' on Twitter
British number one Anne Keothavong will not compete again until 2010 after a scan revealed she has suffered a serious knee injury.
Keothavong, 25, tore both the anterior cruciate ligament and the meniscus in her left knee while playing doubles in Stanford, California, on Tuesday.
She was injured trying to avoid a fence while chasing down a drop shot at the Bank of the West Classic.
"I will have to rehab for the rest of the season," said Keothavong.
"Of course I'm disappointed to be out for the rest of the season but I'll continue to work with my team and look forward to coming back next season."
The Londoner, who was playing with Japanese partner Ayumi Morita against Julie Coin and Marie-Eve Pelletier, had lost the first set 6-4 but was 5-3 up in the second when she was injured.
"I was running towards the fence for an angled shot. I tried to avoid the fence and in doing so I twisted my knee," added Keothavong, who was beaten 6-1 6-4 by world number four Elena Dementieva in the first round of the singles on Monday.
"I'm flying to London to consult with my team of doctors and physios."
Keothavong had earlier posted on her Twitter web page: "Busted my knee, got a scan in the morning but think I know already."
The British number one also posted a picture of her leg bandaged from thigh to toe.
It is the second time Keothavong has suffered a serious knee injury in the USA, and she was out for eight months after sustaining serious ligament damage during an ITF event in Lexington in 2004.
This latest injury comes as a major blow after a year that has seen her break into the world's top 50 for the first time.
Currently ranked 54th, Keothavong would have been defending the ranking points from last year's run to the third round of the US Open and two ITF tournament wins before the season ends in November.