Her victory was the most surprising of the afternoon session on Day 2 of the year's last Grand Slam tournament. Another American teen, Taylor Townsend, was set to face two-time defending champion Serena Williams at night, after Roger Federer played Marinko Matosevic.
Aside from the Williams sisters, American tennis fans have not had much reason to cheer for their own in the latter stages of major tournaments in recent years. That's why the occasional run by a young U.S. player - think Melanie Oudin, wearing ''Believe'' on her sneakers in 2009 - stirs interest at Flushing Meadows.
''Believing was the No. 1 thing that I had to do today,'' said Bellis, whose nickname CiCi is derived from the initials of her first and middle names, Catherine Cartan. ''That's what my coach told me before the match also: Just go out there and believe that you can win.''
Bellis is still young enough that she plans on playing in the junior tournament in New York. She lost in the third round of that event a year ago; she lost in the first round of the Wimbledon girls' tournament last month.
But on Tuesday, there she was in front of a supportive, overflowing crowd on Court 6 at Flushing Meadows, taking it to Cibulkova, the runner-up at the Australian Open in January.
''I went into the match thinking it was going to be such a great experience,'' said Bellis, whose rapid-fire sentences tend to finish with a gurgle of laughter, ''but I never thought I would come out on top winning.''
Neither did her parents. Her mother, Lori, didn't attend the match because she gets too nervous watching in person, Gordon Bellis said. So Lori stayed at their hotel, getting updates via text from Gordon.
''She asked how bad it was going in the first set. And so I told her (CiCi) actually won the first set. She couldn't believe that,'' Gordon said. ''We were just thankful that she didn't get double-bageled. That was the first objective.''
After trailing 3-1 in the third set - ''kind of a bummer,'' Bellis would say later - she came back to even things, then broke the 25-year-old Cibulkova in the final game. When it ended, Bellis crouched at the baseline and pumped her fists, then ran over to the stands for congratulatory hugs.
''I didn't expect her to play so aggressive today. I thought she's kind of a player who's just putting the ball back and running around,'' Cibulkova said. ''That didn't happen today. She was really aggressive on the court and that maybe surprised me in the first set.''
Bellis was making her main-draw tour-level debut, having earned a wild-card invitation from the U.S. Tennis Association by winning the USTA Girls' 18s National Championship, the youngest to do since Lindsay Davenport also won it at 15 in 1991.
Not since Anna Kournikova was 15 in 1996 had someone so young won a match at the U.S. Open. And not since Mary Joe Fernandez was that age in 1986 had an American done it.
''Now it's time for her to refocus, not get too excited about this first round. Just get back to business,'' said Fernandez, an ESPN analyst.
Next up for Bellis is a second-round match against 48th-ranked Zarina Diyas of Kazakhstan. And Gordon Bellis said Lori might decide to attend that one. No word on what sort of rules Mom will have to follow.