NEW YORK: Ask Serena Williams what could prevent her from completing tennis’ first true Grand Slam in more than a quarter-century at the US Open, and the response will not include the name of a single possible opponent.

“Well,” she began when that question was posed, “I’m always one of my biggest competitors. I can always stop myself. So that’s why I kind of just try to stay positive on the court and stay really focused and stay as calm as I can.”

What else?

“I have to make sure I'm good physically,” Serena said.

Anything more to worry about?

“Fear and doubt can stop me, too,” she continued. “If I step out on the court and I'm a little nervous or I'm fearful, then that's never a really good sign.”

Really? That happens?

“It does happen. But I just embrace it and I bottle it up and I throw that bottle away,” Serena said. “And I just go for it.”

Already considered by many the greatest women’s tennis player and among the greatest athletes ever — no matter the gender, sport or era — the 33-year-old American has become quite adept at discarding that fear and doubt.

When the US Open begins Monday, the No. 1-seeded Williams will embark on a bid to become the first tennis player since Steffi Graf in 1988 to win the sport's four most prestigious tournaments in a single season.

Only five players have pulled off a calendar-year Grand Slam; the last man to do it was Rod Laver in 1969.

“Just the fact that she’s gotten herself into that position is an achievement on its own,” said Maria Sharapova, who is seeded third and could face Williams in the semi-finals in New York.

“Her consistency and her level of play throughout this year, and in previous years and her career, have spoken for itself.”

Serena will be the US Open top seed for a fourth time, having already won titles as top seed in 2002, 2013 and last year, and the question remains: Who can beat her?

Second seed Simona Halep of Romania — who could be the toughest healthy foe in Serena’s path — is 1-6 against the big-serving American.

Russian Sharapova, Danish fourth seed Caroline Wozniacki and 2012 and 2013 US Open runner-up Victoria Azarenka of Belarus all have leg injuries.

Sharapova, who has won each of the Grand Slams during her career, is 2-18 against Serena with her last win coming 11 years ago while Wozniacki has won once in 11 meetings with the big-serving world number one.

“What Serena's doing just now is incredible,” said two-time major champion Andy Murray, who joins No 1 Novak Djokovic and No 2 Roger Federer as the favourites for the men’s title.

Serena has been invincible yet has seemed beatable during this late career surge.

The biggest server the women's game has ever known has struggled at times with her delivery and lost nine sets during her victorious run through the 2015 Australian, French and Wimbledon championships, including seven first-set losses.

Yet no one has succeeded in derailing her Grand Slam charge.

“Who can stop her?” pondered US Fed Cup captain Mary Joe Fernandez, a two-time Grand Slam doubles winner and a commentator for ESPN. “If she plays her best, she's better than everybody and she will win. I think the best chance to upset Serena is earlier rather than later.”

Several dangerous players could test Serena early in the Flushing Meadows fortnight.

Mirjana Lucic-Baroni of Croatia, who upset Halep at Roland Garros, figures to be her second-round opponent.

American Sloane Stephens could be there for the third round, and big-serving compatriot Madison Keys or Poland’s Urszula Radwanska may be waiting in the fourth round.

The quarter-finals could put Serena against either her sister Venus, eighth-seeded Czech Karolina Pliskova or dangerous 18-year-old Belinda Bencic of Switzerland.

Bencic beat Serena in a tight three-setter this month in the Toronto semi-finals on her way to a second career WTA title.

Federer eyeing a win for the ages

The US Open has long been a tournament for the 20-something set but an in-form Federer is looking to break that trend.

In the last three-plus decades only Pete Sampras in 2002 and Jimmy Connors in 1983 won the men’s side of the tournament at Flushing Meadows in their 30s. Both were 31.

Now here comes 34-year-old Federer, fresh off an impressive win on the hard-courts in Cincinnati. Even though he is seven years removed from his last Flushing Meadows triumph, Federer has been buoyed from an equipment change and his work with two-times US Open champion Stefan Edberg.

“I would like to give Stefan a lot of credit and he has helped me in a big way,” said Federer, who won five straight US Open titles between 2004 and 2008.

“But I think the backhand is better since I also have a bigger racket head. I really think that was the change because it happened at the same time.”

The 17-times Grand Slam champion moved into second spot in the world rankings with his win over Novak Djokovic in the Cincinnati final, which means he will not face the Serbian world number one unless the rivals reach the Sept 13 final.

“Number two, three doesn't really matter,” the Swiss maestro said. “To be quite honest, I don't think that far ahead because I haven't been in a final since 2009 [when he lost to Argentine Juan Martin del Potro].”

Last year, Marin Cilic of Croatia claimed his first Grand Slam title with a straight sets win over Japan’s Kei Nishikori. There has not been a successful defence on the men's side at Flushing Meadows since Federer's run.

As the ninth seed, Cilic is on Djokovic’s side of the draw and could face fourth seed Nishikori in the quarter-finals.

Djokovic already has the Australian Open and Wimbledon titles under his belt in 2015, taking his majors total to nine.

The US Open champion in 2011, Djokovic was runner-up in 2012 and 2013 before suffering a shock semi-final loss to Nishikori 12 months ago.

The 28-year-old boasts a 56-5 record this year but two of those defeats have come this month — to Murray in the Montreal Masters final and then against Federer in Cincinnati.

Djokovic begins on Monday against Brazilian world number 89 Joao Souza, who has never won a main draw match at a Grand Slam.

“I set myself up for high standards because of the results I've had the last couple of years. I'm an ambitious guy and I devote myself completely to this sport,” said Djokovic.

Murray, the world number three, won in New York in 2012, ending Britain’s 76-year wait for a Grand Slam men;s singles champion.

He has the most difficult draw of the three because he will meet un-seeded Australian Nick Kyrgios in the first round and possibly French Open champion and fifth seed Stan Wawrinka in the quarter-finals.

As the eighth seed, two-time champion Rafa Nadal has a potential clash with Djokovic in the quarter-finals.