Sore knee sidelines Serena

World No 1 Serena Williams has pulled out of upcoming WTA tournaments at Indian Wells and Miami because of a knee injury, organizers said on Tuesday.

A statement issued by BNP Paribas Open organizers in California quoted Williams as saying she was withdrawing because of a problem with her left knee.

"Sadly, I have to withdraw from the BNP Paribas Open in Indian Wells and the Miami Open," Williams said.