Osaka, 21, has since risen to the top of the rankings and won two Grand Slam singles titles: last year’s United States Open and this year’s Australian Open.

Gauff, born and raised in Delray Beach, has just begun to make an impact at this level. She reached the fourth round in her first Wimbledon as a qualifier before losing to the eventual champion, Simona Halep. She is now in the third round of her first United States Open after defeating the tour veteran Timea Babos in three deeply diverting sets on Thursday night and then getting serenaded by the near-capacity crowd at Louis Armstrong Stadium with an extended chant of “Coco.”

“This is just the beginning, I promise,” she said in response, looking much more emotional than she had during the match. “I promise to always fight for you guys, and I thank you so much for all the support. You got me through the past two matches, and I hope you can help me on Saturday and in my doubles tomorrow.”

Her new fans answered the call Friday, a crowd filling the stands and the walkways surrounding Court 5, in the shadow of Ashe Stadium. In their first Grand Slam doubles match together, Gauff and her partner, Caty McNally, a 17-year-old American, saved four set points in the opening set and defeated two fine doubles players, Katerina Siniakova and Julia Görges, 7-6 (6), 6-2.

Gauff and McNally — who in singles pushed Serena Williams to three sets in the second round — have yet to lose a match at any level together in doubles; they won the WTA title in Washington a month ago.