Gauff, already the youngest competitor ever to qualify at the All England Club in the professional era, showed the poise and power of a much older, much more experienced player, pulling off a 6-4 6-4 victory in the first round over Venus Williams, who at 39 was the oldest woman in the field.
When it ended, Gauff dropped her racket and put her hands on her head. After a handshake and exchange of words at the net with Williams, Gauff knelt by her sideline chair and tears welled in her eyes. Up in the stands, her father leaped out of his seat.
"Honestly, I don't really know how to feel. This is the first time I ever cried after a match. Or winning, obviously; I've cried after a loss before," said Gauff, who is based in Florida. "I don't even know how to explain how I feel."
This was her third tour-level match; Williams has played more than 1000. This was Gauff's first at Wimbledon, where Williams has played more than 100 and won five titles. By the time Gauff was born in 2004, Williams already had spent time at No 1 in the ranking and owned four of her seven grand slam singles trophies.
It was by far the most anticipated match of day one at the grass-court tournament, but hardly the only upset. Two-time major champion Naomi Osaka, who was No 1 until a week ago, lost 7-6 (4) 6-2 to Yulia Putinseva, joining two young members of the men's top 10, No 6 seed Alexander Zverev and No 7 seed Stefanos Tsitsipas, on the way out.
Gauff came into the week outside the top 300 but was granted a wild card by the All England Club for qualifying. She rolled through those rounds at a nearby site, knocking off the event's top seed.
But this was a whole other task: a main-draw match against an idol on No 1 Court, with its seating capacity of more than 12,000.
Gauff was sensational and showed zero signs of the moment or the matchup being too daunting for her. It's the sort of unusual calm and steady way she has progressed through the various levels of youth tennis, including reaching the US Open junior final at 13 and winning the French Open junior title at 14.
The first set was remarkable: Gauff had 10 winners to only two unforced errors, all the while trading powerful groundstrokes at the baseline with Williams, and never facing a break point.
"The sky's the limit," Williams said. "It really is."
Gauff, who is black, long looked up to Williams and her younger sister, Serena, the first African American women since Althea Gibson in the 1950s to win a grand slam singles championship.
Asked about Gauff over the weekend, Serena said she reminds her of Venus.
When they spoke after Monday's (Tuesday NZT) match, Gauff said she thanked Venus "for everything she did."
"I wouldn't be here if it wasn't for her," said Gauff, who joined the crowd in applauding for Venus as she walked off the court. "And I was just telling her that she's so inspiring. Like, I always wanted to tell her that. And even though I met her before, I guess now I have the guts to."
She showed plenty of grit in this match, particularly after getting broken to make it 4-all in the second set. Gauff steadied herself right there, though, breaking right back with a pair of forehand passing shots that drew errant volleys.
And then in the final game, Gauff needed to erase the disappointment of wasting her initial three match points. She did just that, converting her fourth when Venus put a forehand into the net.
"I never thought this would happen," Gauff said. "I'm literally living my dream right now. And not many people get to say that."