A spiralling injury list on the women's tour has left the US Open in danger of being robbed of many of its major players.

With the final Grand Slam of the year set to get underway on August 25, champion Serena Williams heads a casualty list that is threatening to leave the Open with a seriously depleted women's field.

Five of the WTA's top eight players and nine of its top 20 competitors have pulled out of tournaments during the past three weeks.

"I don't think I have ever seen it this bad," said former top-five player turned television pundit Pam Shriver.

The JP Morgan Classic in Los Angeles lost its top draw after world number one Williams had knee surgery last week.

Williams is expected to be sidelined for six to eight weeks and will not defend her US Open title.

Having grown up just a few miles from the tournament's site, the American was counted on to sell tickets at the JP Morgan Classic.

But only 1,814 fans attended Saturday's semi-final between Lindsay Davenport and Japan's Ai Sugiyama in a new stadium that seats 8,000 people.

"In markets like Los Angles and New York, you really need stars to sell tickets," said Shriver.

"I'm really concerned about the US Open because as an American, I like to see our players do well.

"But at this point, the US Open is in danger of waiting to see which Belgian (Kim Clijsters or Justine Henin-Hardenne) will win the title."

Seventh-ranked Jennifer Capriati joined the casualty list during last week's Acura Classic with a pectoral (chest) strain and was forced to retire midway through her second-round match.

Davenport skipped last month's tournament in Stanford with a left foot injury.

DEMANDING SCHEDULE

Next week's Canadian Open in Toronto has also been hit with injuries.

Serena's older sister Venus has withdrawn because of an abdominal strain while Monica Seles, Chanda Rubin, Meghann Shaughnessy, Alexandra Stevenson, Patty Schnyder and Anna Pistolesi are the other members of the top 30 to drop out.

Venus has not played since losing the Wimbledon final to Serena in early July and her next scheduled tournament is the US Open in New York.

Eighteen-times grand slam singles winner Martina Navratilova blamed the injuries on the players' demanding schedule.

"If you look at the number of tournaments people play now and average it out against the number of tournaments the top 10 played 10 years ago, they probably play five more tournaments a year now," said Navratilova, who was playing doubles in Los Angeles with Svetlana Kuznetsova.

"That's a lot of matches and time you're not taking care of your body or recovering from injuries.

"Serena and Venus have pulled out of more tournaments in one year than I did in my whole career.

"I don't think they play enough and the other top players play too much. The other top players play between 16 and 18 tournaments and that's too hard mentally and physically."

OVER TRAINING

Along with Seles, who has not played competitively since her first-round loss at the French Open in May, Russian pin-up Anna Kournikova has also been out of action for months.

A long-term back injury could also rule out Kournikova from the US Open.

Among the 10 withdrawals from this week's JP Morgan Classic were defending champion Rubin (shoulder injury), former winner Seles (foot problems), Russia's Vera Zvonareva (elbow injury) and American teenager Ashley Harkleroad (elbow injury).

Capriati suspects over training has contributed to the spate of injuries.

"It's a long season and, with the grind of the tour, there's not much time to take a break, let alone recover" said Capriati.

"I don't feel like I can sit back and not try to play. Plus, the game is so much more powerful now and maybe some of the girls are over-training trying to get stronger."

Having suffered a plethora of injures over the last three years, former French Open champion Mary Pierce said: "I was over-training all the time.

"You have to learn that you can only do so much in practice. I probably hit three times as many balls than any other girl from age 10 to 18. Now I have no desire to do so."

Navratilova added, "The kids play too much on hard courts at an early age and their joints suffer.

"When the joints get messed up, the muscles follow. Most tennis injuries are related to joints because of the hard pounding. Tennis is a very hard sport on your body."