AP Photo Cori "Coco" Gauff (right) greets Venus Williams at the net after upsetting the former champion in their women's singles match during day one Monday.

WIMBLEDON, England — Coco Gauff grew up admiring the Williams sisters. Picked up a tennis racket as a little girl because of them. And on Monday at Wimbledon, still just 15, Gauff beat one of them.

Gauff, the youngest competitor 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 1,000. This was Gauff’s first match 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 rankings and owned four of her seven Grand Slam singles trophies.

“It didn’t really seem real, for a moment,” said Gauff’s father, Corey, between handshakes and slaps on the back and requests for selfies from spectators leaving No. 1 Court. “On the walk to the court, I was walking behind her. She was excited. I was excited. She seemed confident, but I wasn’t sure if it was false confidence or she really was. I just said to her: This match is really magical. Just enjoy it. Your first Wimbledon main draw and you’re on a main court against somebody you looked up to from the beginning.”

It was by far the most anticipated match of Day 1 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 certainly has the mindset of someone who intends to go far.

“I’ve said this before: I want to be the greatest. My dad told me that I could do this when I was 8. Obviously you never believe it. I’m still, like, not 100 percent confident. But, like, you have to just say things. You never know what happens,” she said. “If I went into this match saying, ‘Let’s see how many games I can get against her,’ then I most definitely would not have won. My goal was to play my best. My dream was to win. That’s what happened.”