In a less-traveled part of the 46-acre Billie Jean King National Tennis Center, an imposing black metal fence serves as a reminder of the multiple security worries that come with hosting the U.S. Open.

New York police counterterrorism officials insisted that the fence, near a new 8,000-seat grandstand stadium, be reinforced earlier this year with thick strands of cable capable of withstanding a crash from a truck loaded with explosives.

"A lot of this stuff is off the radar, but it goes back to what's happening in the world," the event's security director, Michael Rodriguez, said during a flurry of last-minute preparations for Monday's opening matches.

Terrorist attacks in Europe and mass shootings in the U.S. have only added to vigilance over this year's U.S. Open, which already posed daunting security challenges because of its sheer size: 700,000 spectators over two weeks packed in stadiums and smaller venues connected by broad pedestrian walkways. And this year's event just happens to end with the men's final on the 15th anniversary of the Sept. 11 terror attacks.

Officials say they know of no credible threats against the U.S. Open. But the NYPD, which ordinarily has hundreds of officers in and around the tennis center, plans to increase its presence this year. That's on top of the tournament's roughly 300 private security guards.

Authorities have also installed temporary closed-circuit surveillance cameras, including some perched atop the 23,771-seat Arthur Ashe Stadium, the event's largest venue, along with devices to detect chemical, biological or radiation risks.

"I've already warned people they're going to see it, and they should feel good about seeing it," said Rodriguez, a former NYPD detective sergeant and supervisor for the Joint Terrorism Task Force.

Sally Kane, 71, of New Rochelle, New York, one of the many fans to turn out last week for free-admission qualifying matches, scoffed at the potential risks, saying, "Terrorists don't want to come here." She was annoyed that she had to pay to store her metal water bottle outside the center.

"I would have felt just as safe with my water bottle," she said.

Rodriguez said he meets twice a day with representatives from the various local and federal law enforcement agencies monitoring the U.S. Open. Sample topic: the difficulties of "drone mitigation."

Such widening risks were underscored last year when a small drone crashed into the stands during a match (It turned out a science teacher lost control of it while flying it in a nearby park) and in 2014, when an NYPD intelligence analyst told a gathering of private security directors that an al-Qaida online publication had encouraged a car-bomb attack on the tournament.

The NYPD will keep heavily armed officers trained to respond to terror attacks at the ready near the grounds. As with other events like New Year's Eve in Times Square, plainclothes officers will mix with the crowds inside and outside the site.

Other defenses include the screening checkpoints for the throng of spectators. In the past decade, the U.S. Open became among the first sporting events to prohibit backpacks and use airport-style, walk-through metal detectors instead of less-reliable wands to check people for weapons.

The U.S. Open also has a rigorous inspection system for the more than 1,600 trucks that make deliveries during the event. Drivers are vetted, given a bar code and directed to an inspection point deliberately located a quarter-mile from the tennis center before they can enter the grounds.

Last week, United States Tennis Association officials held a meeting at the tennis center with their counterparts from the French Open and other major tournaments to discuss ways to keep spectators safe.

"Anything can happen," Rodriguez said. "The question is, what are we doing to minimize the risks."