The return from injury of Serena Williams and the return to form of Maria Sharapova has added a sense of predictability to a U.S. Open women's tournament overflowing with uncertainty.

The former world number ones are favourites to win the last Grand Slam of the year, partly because they rediscovered their touch at the right time but mostly because it is a period when the women's game has never been more open.

Unlike the men's game, which has long been at the mercy of an elite group, there are few certainties about women's tennis right now and the world rankings offer few reliable guidelines to finding the winner.

World number one Caroline Wozniacki has still not won a Grand Slam and the Dane has played in just one final, at the U.S. Open two years ago.

The world number two, Russia's Vera Zvonareva, is also chasing her first Grand Slam title after making the finals at Wimbledon and the U.S. Open last year.

Victoria Azarenka of Belarus, the fourth seed at Flushing Meadows, has only once even made the semi-finals of a Grand Slam, meaning three of the top four ranked players for the Aug. 29-Sept. 11 tournament have yet to win a major.

The lone exception is Sharapova, who won Wimbledon in 2004, the U.S. Open in 2006 and the Australian Open in 2008.

The 24-year-old has been plagued with health problems in recent years but returned to near her best this year, reaching the Wimbledon final and winning last week's Cincinnati Open.

Williams, the most dominant player of her generation, missed last year's U.S. Open as part of an 11-month layoff caused by injury and health problems.

The American returned to the courts in June and captured back-to-back tournaments in California and Toronto to climb back to 29th in the rankings.

She was bumped up one place to 28th seed after last year's U.S. Open champion and current world number three Kim Clijsters withdrew because of a stomach muscle injury, and remains a master of peaking for the big events.

"It's center stage, it's New York, it's the last Grand Slam of the year," Williams said.

Clijsters provided one of the great fairytales of tennis when she won the U.S. Open for a second time in 2009 after quitting the sport to start a family.

She successfully defended her title last year and won the Australian Open in January but injuries slowed her down and eventually caused her to pull out this year.

Women's tennis has been waiting a long time for the arrival of the next generation and while there have been some new Grand Slam winners in recent years, it has been the experienced players rather than brash newcomers who have come through.

Italy's Francesca Schiavone won last year's French Open and China's Li Na won in Paris this year. The Wimbledon champion this season was Czech Petra Kvitova.

All three are seeded in the top eight for New York but have not played with enough consistency to climb higher, leaving the U.S. Open wide open.