Elbow keeps Serena out

World number one and defending champion Serena Williams withdrew Friday from next week's WTA hardcourt tournament in Stanford with a right elbow injury, organisers said.

"I'm disappointed to have to withdraw from the Bank of the West Classic, but I need to get my elbow back to 100 per cent," Williams said in a statement released by tournament officials.

The 33-year-old American had planned to start her build-up to the US Open in Stanford.