(updates with quotes)
MADRID Nov 7 (Reuters) - Serena Williams is hoping to play on at the WTA Championships despite having to retire through injury after the first set against Russian Anna Chakvetadze on Wednesday.
The 26-year-old American called for the doctor to attend a problem with her left knee when the score was 5-4.
After having some strapping, she lost the set 6-4 but Williams looked short on mobility and was unable to continue her opening match as the second set got underway.
"Before the match I had taping for prevention since I was feeling a little tight in both knees. I thought I was tighter in the right knee but then, once I had to increase my range of movement, the left knee got worse," Williams said in a statement.
"I've had issues with my knees before especially the left knee since that's the one I had surgery on in 2003. It's unfortunate it's happened now and I can't stress how disappointed I am.
"Right now it's swollen and I'm going off to have an MRI at the hospital to see exactly what the problem is. It is my intention to play on in this event."
Williams's withdrawal from the match means world number one Justine Henin, who has won her first two outings in the yellow group, has automatically qualified for the semi-finals.
If the 2001 winner is unable to continue, Marion Bartoli of France is on standby to step in.
(Editing by Ken Ferris)