Knee injury forces Serena out of Miami

World number one and Australian Open winner Serena Williams withdrew from the Sony Ericsson on Friday as she continues to nurse a sore knee, becoming the fifth high profile player to pull out of the event in the last week. "Obviously, we are disappointed when any of the top players pull out,'' tournament director Adam Barrett said. "When you are talking about one of the most prolific players in the game, a player like Serena, it is extremely disappointing, especially for the fans who won't get to see her.

"We want the best players versus the best players, but we still have a great field with many of the top players in the world. Only difference is this year, we know Serena won't win it. But if a player isn't at 100 percent and can't play their best, it's better for them to sit out. We're here for the long haul. I'd rather see Serena have a long career than have her play in Miami two weeks and jeopardize her health.''

Maria Sharapova and world number two Dinara Safina pulled out on Wednesday, along with 2008 champion Nikolay Davydenko. Tournament organizers also announced the withdrawal of Juan Martin del Potro, a semifinalist in last year's tournament. Williams won the Key Biscayne event five times and was coming off her fifth Australian Open victory in January. Sister Venus Williams will take Serena's spot in Thursday's match that starts at 7 p.m., as she looks for her fourth Sony Ericsson Open title. The seven-time Grand Slam champion last won in Miami in 2001.