More NYPD officers and a larger U.S. Open security force are patrolling at the newly renovated Flushing facility with officials citing terrorist attacks in Europe for the added vigilance.

On the venerable tennis tournament’s opening day of competition, fans entering the grounds of the Billie Jean King National Tennis Center saw more security as well the results of an ongoing facility face-lift.

The NYPD has deployed more officers from its Counter Terrorism Unit and a Critical Response Command, formed last year after the terrorist attacks in Paris, France.

The two-week Grand Slam tournament is expected to draw more than 700,000 to the Billie Jean King National Tennis Center in Queens. NYPD officers with “heavy weapons,” — some in plain clothes, others undercover, will be on patrol, officials said.

“With the creation of the Critical Response Command, we have a significantly larger counterterrorism footprint at the U.S. Open this year, and we’re able to provide much more highly trained and equipped tactical assets for the Open to provide security,” said NYPD Deputy Chief Scott Shanley, commanding officer of the Critical Response Command.

Evidence of stepped-up security, including more uniformed NYPD officers strolling the grounds toting imposing-looking semi-automatic weapons, wasn’t the only noticeable change at the tennis center.

Throngs of fans got their first look at the gleaming new retractable roof over Arthur Ashe Stadium. The roof is among several upgrades this year.

Fans first had to make it past a gauntlet of security workers enforcing the Open’s strict rules on what is allowed in. Next week, officials expect to add X-Ray technology to screen arriving fans.

Mike Rodriguez, security director for the United States Tennis Association and U.S. Open, said there is a focus this year on areas outside the tennis center where visitors line up to enter. Rodriguez referred to it as the “secondary perimeter.”

“This is always an attractive target” for a “would be terrorist,” Rodriguez said. “It’s where people gather.”

Rodriguez pointed to last November’s explosions outside a soccer stadium during a terrorist attack in Paris as an example of the threat to sporting events. In 2014, a terrorist magazine encouraged bombings at the U.S. Open, according to news reports.

“As the threat changes, how we measure to the threat has to be changed,” Rodriguez said. “With what the latest issues are around the world, we have to adjust.”

Shanley said his department’s strategy includes a shift to more counterterrorism officers, who are stationed at the U.S. Open with the NYPD’s patrol officers.

“The biggest concern is a mass casualty,” Shanley said.

Security planning occurs months in advance of the Open and tournament officials are receiving briefings from the FBI, Department of Homeland Security, and the NYPD.

“People are concerned about what’s happening out in the world,” Rodriguez said.