New York: Serena Williams has been vulnerable this grand slam season but unbeaten in the year's first three major championships.

With the 33-year-old American looking to register the first Grand Slam in 27 years with a triumph at the Aug. 31-Sept. 13 US Open, the question remains: Who can beat her?

Second seed Simona Halep of Romania is 1-6 against the big-serving American, who has won the last three U.S. titles.

Third-seeded Russian Maria Sharapova, who has won each of the grand slams during her career, is 2-18 against Williams with her last win coming 11 years ago.

Fourth seed Caroline Wozniacki of Denmark has won once in 11 meetings with the big-serving world number one.

Williams, winner of the last three U.S. Open titles, 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 U.S. 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 Williams early in the Flushing Meadows fortnight.

Mirjana Lucic-Baroni of Croatia, who upset Halep at Roland Garros, figures to be Williams' 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 Williams against either her sister Venus, eighth-seeded Czech Karolina Pliskova or dangerous 18-year-old Belinda Bencic of Switzerland.

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

‘You sort of need a Bencic (type) that goes out there bold and brave, loving the moment and playing Serena,’ Fernandez told Reuters.

Victoria Azarenka has given Williams stiff challenges at the Open in the past, but the 20th-seeded Belarussian is in the other half of the draw and could only face her in the finals.

Halep, who reached two finals in the U.S. hardcourt run-up to the Open, has won more hardcourt matches this year than anyone on tour with 32 and is tied with Williams for most WTA titles on the surface this year with three.

‘I have a lot of confidence right now because I played tough matches and won difficult matches,’ said Halep said. ‘I'm pretty sure I have a good game now.’