Serena withdraws from Miami tournament

Serena Williams pulled out of next week's Nasdaq Open in Miami on Friday, citing a knee injury and lack of training.

Williams, who has not played since losing in the Australian Open third round in January, has been fined heavily by the WTA Tour for pulling out of several tournaments this year.

She underwent knee surgery in 2003 and has not won a title since taking her seventh grand slam crown at the 2005

She has only played in nine tournaments since and is currently ranked 58th in the world, her lowest mark in seven years.

"I am very disappointed to withdraw but I feel like I need more time before I am able to compete at the highest level," Williams said in a statement.

Williams won the tournament three years in a row from 2001 and lost to her sister Venus in last year's quarter-finals.