Before the official tennis matches begin at the U.S. Open in Queens on Monday, there is practice.

And the real die-hard fans started filling the USTA Billie Jean King National Tennis Center in Flushing Meadows on Sunday, seeking autographs, selfies with the players, or just a real-life glimpse of the tennis elite.

“You have to hunt them down,” said Nir Paz, 42, of Hewlett, who by early afternoon had scored American singles player John Isner’s signature on his basketball-sized neon green tennis ball, a popular souvenir among the autograph-seekers.

Paz was unfazed when Marin Cilic of Croatia (and an Isner opponent), whizzed past, flanked by security as he made his way to the practice courts without stopping for fans.

“It was his people,” Paz said of the security guards. “Not him.”

Besides the usual excitement over the players, this year’s U.S. Open brings a slew of new facilities and amenities, including a retractable roof on the 23,771-seat Arthur Ashe stadium, to be used in the case of inclement weather. A new 8,125-seat Grandstand stadium also opens, as well as several outdoor courts and eateries.

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But on the eve of play, the real excitement was derived from the mere presence of the athletes.

“Oh that’s Venus — Serena’s sister! Sitting right there!” a young boy shrieked, running to catch a glimpse.

It was, in fact, Venus Williams, taking a quick break as she practiced Sunday morning in the blazing sun. She signed a couple autographs before quickly making her way into the air-conditioned venue.

A lesser-known player, France’s Kristina Mladenovic, was caught up in a mini-swarm of fans after a round of practice, but good-naturedly obliged for several photos.

Ganesh Krishnamurdhy, a competitive tennis player from Singapore, scored a selfie with Mladenovic as he happened to be walking by.

“My dream is to take a picture with Roger,” said Krishnamurdhy, referring to tennis great Roger Federer.

Dozens of fans, squished between medal barricades, stood for hours in the heat, and were disappointed when Spain’s Rafael Nadal bypassed them and took a different exit after practicing.

“Rafa!” they shouted, hoping he’d change course. But Nadal slipped into a silver sport utility vehicle and was gone.

Still, Sowbi Krish, 19, of Montville, New Jersey, was unfazed.

“He’s just a hard worker,” she said of Nadal. “He gives everything on the court. And he’s such a role model. He keeps coming back after injuries. He never gives up.”