“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,” said Bellis, who is ranked 1,208th. “I’m still in shock about that match.”

Bellis, who is known as CiCi, was not alone in that sentiment. Earlier Tuesday, 17-year-old Borna Coric of Croatia, the youngest player in the men’s draw, toppled No. 29 seed Lukas Rosol, 6-4, 6-1, 6-2.

Image Madison Keys, 19, seeded here for the first time, won her opening match. Credit... Earl Wilson/The New York Times

“I came here and I was only thinking about getting in the main draw,” said Coric, who made the tournament as a qualifier. “That was my first goal. I said to myself, If I achieve that, that’s going to be perfect. Now actually winning a guy who is top 30 or close, you know, it’s just unbelievable.”

But as young as Coric, the Open junior champion last year, is, Bellis did him one better.

She had earned a wild card into the tournament by being the youngest girl to win the girls’ 18s national championships since Lindsay Davenport won as a 15-year-old in 1991. The last 15-year-old to win a match at the Open was Anna Kournikova in 1996. Bellis was not even born until 1999.