US Open tournament director Jim Curley has promised to increase security for players after a fan rushed at Rafael Nadal.

Nadal had just defeated Gael Monfils to reach the quarter-finals at Arthur Ashe Stadium and was changing shirts when a man with a camera jumped over an advertising hoarding and onto the court and kissed the smiling Spaniard on the cheek.

Nadal appeared unperturbed by the incident as security guards quickly pounced on the man and led him away.

It was the second such incident this year at a grand slam event after a fan got on the court at Roland Garros during the French Open final in June, and got close to Roger Federer.

Again, the man appeared harmless, waving a flag and trying to place a hat on the world number one's head, but the intrusion brought back memories of a 1993 attack on Monica Seles in Hamburg when the world number one was stabbed in the back by an obsessed Steffi Graf fan.

"It's important to understand that the safety of our players, our fans and everyone on site here at the National Tennis Centre is first and foremost in our minds and what took place last night was simply unacceptable," Curley told the ESPN Sports television channel.

"We do have a great security system in place, the head of security is a member of the joint terrorism taskforce and other sporting bodies come to us to learn how to handle situations such as this.

"Having said that, what took place last night was unacceptable and we train not only prior to the event but during the event, every day, to make sure these things don't happen.

"Unfortunately it happened and I can assure everyone that we are going to do everything within our power to make sure it doesn't happen going forward."

Curley confirmed there would be more player protection in place for the rest of the tournament, which ends on Sunday.

"I can assure you that will be the case," he said. "It was a breakdown, no doubt about it, and we're going to correct it."