Who will avoid the injury-bug influenza?

Editor's note: Beginning Aug. 16, Ravi Ubha is unveiling the top 10 U.S. Open questions. Check back each weekday until Aug. 27 as we count down to No. 1.

4. Who is healthy enough to win?

They're dropping like flies.

After Serena Williams, the women's favorite, pulled out of the U.S. Open with a foot injury, Juan Martin del Potro followed suit, insufficiently recovered from wrist surgery.

Top 10 U.S. Open questions

No. 10: Is Federer the people's choice?

No. 9: Will Nalbandian shine?

No. 8: Which U.S. man steps up?

No. 7: Encore for Kim Clijsters?

No. 6: Which dark horse will go deep?

No. 5: Can Murray win when it matters?

Williams becomes the first women's No. 1 (she is for now) to miss the tournament since rankings were introduced in 1975, and del Potro is just the third man in the 42-year Open era unable to defend his New York crown.

Further, Justine Henin, who played at the Australian Open, French Open and Wimbledon in her comeback, won't compete because of an elbow injury suffered in southwest London. Exciting Frenchman Jo-Wilfried Tsonga can't catch a break. He's gone, too.

Four big names out.

Other contenders and dark horses have their own concerns.

Maria Sharapova might be the co-favorite among the women, with Kim Clijsters. Sharapova skipped the rain-affected Rogers Cup in Montreal courtesy of a left heel injury. Her agent, Max Eisenbud, said in an e-mail, though, that Sharapova was "all good" for the season's final major.

Clijsters was visibly bothered by her left hip in Montreal, and the Belgian's 13-match U.S. Open Series winning streak subsequently ended. Clijsters plans to use tape as support, adding that the current hip injury isn't as painful as the one in 2006.

"I am in no doubt over my participation in the U.S. Open," Clijsters told reporters. "I will likely have some strapping on my leg, but that does not mean that I will not be at full fitness."

Samantha Stosur, a French Open finalist, made her return this week in New Haven, Conn., after an injury to her serving arm.

"A combination of rest and some time in a hyperbaric chamber has the Queenslander ready to begin her U.S. Open assault," the Aussie's website disclosed.

Venus Williams, without a non-grass major since 2001, missed Montreal and Cincinnati with a left knee complaint.

Ana Ivanovic, a former world No. 1 who was drifting, showed signs of life in Cincinnati, reaching the semifinals. Then she hurt her left ankle. Ivanovic withdrew from New Haven, saying she was "feeling a lot of pain." That doesn't sound good for New York.

Russian teen Anastasia Pavlyuchenkova, on a roll since Wimbledon, also withdrew from New Haven, tweeting that her leg and shoulder weren't 100 percent.

Andy Roddick was hindered by a mild case of mono and didn't play in Toronto, but he got the match practice he wanted in Cincinnati. His semifinal showing put him back in the top 10. Fellow American John Isner doesn't know whether he'll be fit after the right ankle injury he picked up in Cincinnati. Even if he does play, what kind of shape will he be in?

Tomas Berdych, finally playing the way we all thought he could, was undone by a right leg injury in Cincinnati.

And there are three months of the season remaining.