Top-ranked Serena Williams, who was going for a record sixth title at the upcoming Sony Ericsson Open on Key Biscayne, withdrew Friday due to a knee injury.

Williams injured the knee during the Australian Open and has not played since.

Defending U.S. Open champion Juan Martin del Potro, a rising Argentine star, also pulled out Friday with a wrist injury.

"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.''

As for Del Potro, Barrett said the Argentine fans in particular will miss out.

"He's a very exciting player and has been a big draw here,'' Barrett said. "The good news is, he's young, and I expect him to come back next year and for 10 or 12 years after that. You're always going to lose some players to injury, but that doesn't diminish the tournament.''

Williams' older sister, Venus, a seven-time Grand Slam champion, will replace her as part of the featured night match at 7 on Thursday. Venus Williams has won three Sony Ericsson Open titles, the last in 2001.

Earlier in the week, former No.‚1 players Maria Sharapova and Dinara Safina, and Nikolay Davydenko all withdrew.

Despite the withdrawals, the tournament features the most impressive field outside of the Grand Slam events. Among the names are: top-ranked Roger Federer, No. 2 Novak Djokovic, No. 3 Rafael Nadal, No.‚4 Andy Murray (the defending champion), defending U.S. Open champion Kim Clijsters, seven-time Grand Slam champion Justine Henin, No.‚8 Andy Roddick and No.‚2 Svetlana Kuznetsova.

Qualifying rounds begin Monday. The women's main draw begins Tuesday (a day earlier than in years past) and the men's main draw begins Wednesday. Federer opens play next Saturday night.

Tickets are available by phone (305-442-3367) or internet at www.sonyericssonopen.com. The two-week tournament ends with the women's final on Saturday, April 3, and the men's final on Sunday, April 4.