For seasoned autograph seekers, the first day of the U.S. Open is prime time to stalk their game: the signatures of their favorite tennis pros.

On Monday, fans by the dozens jostled for position at the Billie Jean King National Tennis Center in Flushing -- including Barbara Klein, 73, of Manhasset, who has been collecting the autographs of tennis players for four decades.

Klein got a front and center spot at a security barrier, where fans begged players coming on and off practice courts for autographs.

"It's a challenge. . . . You're getting banged around. After you get something, you feel accomplished," Klein said.

The first day of the tournament is considered by many fans the best day for autographs because none of the players has been eliminated yet.

At about 12:30 p.m., fans behind the barrier leaned as far forward as possible, yelling in excitement as top-ranked player Novak Djokovic came off the practice court.

"I love you Novak!" cried one fan.

"Can I get a selfie?" another asked, as gleeful fans handed over tournament programs and giant souvenir tennis balls for him to sign.

Ioason Bolivar, 43, of Jersey City got in line at 7 a.m. to make sure he'd get a good spot behind the barrier.

"Every top player in the world is here," Bolivar said.

His efforts paid off when his target, player "Stan the Man" Wawrinka, came by and grabbed a custom T-shirt that Bolivar had made for him.

"I'm so happy. He said thank you!" Bolivar said in awe after Wawrinka walked away.

Rutgers University senior and Massapequa native Jeff Guadagno, 21, got squished against the security barrier when player Eugenie Bouchard walked up. He called out to try to get her attention.

"I like your shirt!" Guadagno yelled out.

"You like my what? My shirt?" Bouchard replied, busy signing autographs.

The college student couldn't grab his phone to take photos in time, but he got his moment with a tennis celebrity.

"It's worth it," he said with a big smile.

Some players, such as Andy Murray, grinned and waved at fans on the way into the practice courts, promising to stop on the way out.

But not every fan was willing to wait it out to get an autograph.

"Mental picture is going to have to do," Michele Brown, 43, of Vienna, Virginia, told her 11-year-old son, Joshua, after Murray's temporary deflection. "Can't wait around all day. Time for some tennis!"