US Open stars see Serena as nearly unstoppable

Serena Williams overcame foot surgery and blood clots on her lungs in the past two years to regain the form that has made her a 14-time Grand Slam tennis champion.

Now her rivals and admirers from men's tennis see her as the unmatched queen of the sport and a clear favorite entering Monday's start of the US Open.

"It's incredible. Nobody knows where she is going to stop and she keeps on going and dominating," said defending US Open men's champion Novak Djokovic.

"I'm happy I'm in men's tennis -- not needing to face her."

Djokovic, the Australian Open winner, is hitting form at the right time -- his runner-up spot in Cincinnati came on the back of a Toronto Masters triumph seven days earlier.

Williams won last month's Wimbledon title, her first Grand Slam crown since her health issues arose after a 2010 Wimbledon title, and followed up by taking the London Olympic gold medal on the same All-England Club grass courts.

"She has had some issues. That makes the return to the top of the rankings and the top of her game even more exciting," said World No. 1 Roger Federer, a 17-time Grand Slam champion.

"It has been great what she has done over the last 15-plus years but it's an amazing summer for her. She played great. If she's on she's very hard to beat. I think she just proved that point again."

Federer aims to cap his dramatic renaissance by becoming the first man in 87 years to win six US Open titles when the season's last Grand Slam event takes place from Monday.

World number one Federer currently has five New York wins, a mark he shares with US legends Pete Sampras and Jimmy Connors, an equal-best performance in the Open era.

But the last man to win six was Bill Tilden, who achieved the feat in the strictly amateur days of 1925 before finishing his career with seven in 1929.

"What we have seen over the last few months is the best player ever," Kim Clijsters, a four-time Grand Slam champion set to retire after the US Open, said about Williams.

But it was a rise from the lowest of depths. Williams needed 18 stitches and surgery after stepping on broken glass in a Munich restaurant in a celebration of her 2010 Wimbledon triumph.

Then she suffered life-threatening blood clots on her lungs and thought her time at the top was over, making her comeback to win at Wimbledon even more astounding and her favored status at Flushing Meadows a certainty.

"She gained a tremendous amount of confidence at Wimbledon," four-time Grand Slam champion Maria Sharapova said. "When she got to the Olympics with every match she just improved.

"She took that confidence and played just really great physical tennis, served extremely well. Obviously she's the favorite because she won those two big events back to back."

China's ninth-ranked Li Na, who started the latest run with her landmark 2011 French Open triumph, is a contender again at the US Open after a Cincinnati title and a runner-up effort in Montreal.

A hefty crowd in the title hunt also includes top seed and World No. 1 Victoria Azarenka of Belarus, the reigning Australian Open champion, and Poland's World No. 2 Agnieszka Radwanska, the Wimbledon runner-up to Serena.

Russian third seed Maria Sharapova, who completed a career Grand Slam with her fourth major title at this year's French Open, won the 2006 US Open and should contend as well.

Andy Murray goes into the US Open, where he was runner-up to Federer in 2008, under a fitness cloud having withdrawn from Toronto after one match with a knee injury and then losing to French lucky loser Jeremy Chardy in his second match at Cincinnati.

Outside of the big three, the likes of David Ferrer, Del Potro and Jo-Wilfried Tsonga will be favoured to pounce in case of a slip-up.

Spanish world number five Ferrer, a semi-finalist at the French Open and quarter-finalist at the Australian Open and Wimbledon, has a best New York finish of the semi-finals in 2007.