NEW YORK – A 22-year-old man stood up and wiggled his hips like a child. Blue cap turned backwards on his head, jean shorts hanging low, he held his smartphone up with both hands. As Catherine "CiCi" Bellis' forehand landed just inside the line, the man stood, continuing to record. There was no time to clap. He needed to keep the camera steady.

"Oh my god, that's definitely an ESPN top 10! Send it!" the man's friend yelled. "She turned 15 this year."

"She was 14 not long ago," another woman marveled out loud.

The time was 5:19 p.m. ET. Serena Williams was just about to take the practice court, a mere 200 feet away, where she'd warm up alongside Taylor Townsend, preparing for their match later Tuesday. Williams herself has dubbed Townsend the "future of American tennis."

Bellis reacts after a point against Dominika Cibulkova. (AP) More

At this moment, though, no one noticed. They were watching the future of American tennis right in front of them. Her name is CiCi Bellis. She's 15. She's ranked 1,208th in the world. Earlier this month, she won the USTA Girls 18's national championships, which earned her a wild card to the main draw here.

On Tuesday, she knocked off 12th-seeded Dominika Cibulkova in the women's singles draw, the first major upset of this year's U.S. Open. Subsequently, she's unable to claim the $60,000 prize money for advancing to the second round because she wants to maintain amateur status.

Bellis came out strong, taking the first set 6-1. She fought hard in the second set, but seemed to be slowing down as she fell 4-6. Fans crammed five rows deep along the short fence that separates the court from an open food court. On the other side, they stood nine rows deep, many using the Court 5 bleachers to watch Court 6. In the bleachers, every seat was packed, with some fans sitting two to a single seat.

After trailing 3-1 in the third set, she seemed to get her footing back. She stopped shanking her shots wide. She started drilling her serves out of Cibulkova's reach. With every point, the fans jumped from their seats.

She held serve to go up 5-4 in the third set. The fans all held their breath. (Well, first they screamed. Then they held their breath.)

As Bellis took the court for what the fans hoped was the final game, a thunderous applause erupted from Court 4, barely 500 feet away. There, American Christina McHale had just defeated Chanelle Scheepers in three sets. Would Bellis follow suit?

She flew through the final game, stopping only to pump her fist after each point. Just like that, the 15-year-old won her first match in her first Grand Slam event.

After the match, Bellis said she was as surprised by the result as everyone else.

"I went into the match thinking it was going to be such a great experience, but I never thought I would come out on top winning," she said. "Words can't describe it right now. Maybe tomorrow I'll be able to think of it better."

Then, she gave some of the credit to the crowd.

"It gave me more energy," she said. "I love it when people watch me. It gives me more energy and makes me play better."

She can be certain a lot of people will be watching her in the second round, where she'll face fellow U.S. Open rookie Zarina Diyas.

More U.S. Open coverage: