Coco Gauff, 15, stops Naomi Osaka at the Australian Open and becomes the youngest player since 1991 to beat a top-5 opponent. (2:01)

American Coco Gauff became the youngest player in the professional era to eliminate the reigning women's champion at the Australian Open, beating former No. 1 Naomi Osaka 6-3, 6-4 in the third round at Melbourne Park on Friday.

After the match, during her on-court interview, Gauff turned into a rather typical teenager, joking about wanting to take "a selfie for Instagram" with Rod Laver, the 11-time major champion after whom the stadium is named.

"Honestly, like, what is my life? Like, oh, my gosh!" Gauff told the crowd. "Two years ago, I lost first round in juniors, and now I'm here. This is crazy."

Gauff is also the youngest player to beat a top-five opponent in a women's tour-level match since American Jennifer Capriati did it at 15 in 1991.

"You don't want to lose to a 15-year-old, you know?" Osaka said.

It was the second significant result of Day 5 in Melbourne: In the same quarter of the bracket, 23-time major champion Serena Williams lost to 27th-seeded Wang Qiang 6-4, 6-7 (2), 7-5.

Gauff pulled this off with some big serving and consistent groundstrokes, and by letting Osaka largely be her own undoing.

Osaka made 30 unforced errors, Gauff merely 17. This was a rematch from the third round of the US Open in September; Osaka won that one in straight sets then consoled a crying Gauff afterward and encouraged her to speak to the fans.

"Her serve is way better," Osaka said Friday. "I feel like I wasn't really swinging freely, and she was."

Coco Gauff celebrates her victory over defending champion Naomi Osaka at the Australian Open. William West/AFP

Osaka made her own rapid ascent to the top of tennis, claiming trophies at the US Open in 2018 and Australian Open in 2019 to rise to No. 1 in the WTA rankings. She is only 22.

That seems old by comparison, of course.

There were the occasional signs that Gauff is not a fully formed player just yet. For example, leading by a set and a break, serving at 1-0, 40-15, Gauff double-faulted twice to get broken for the first time. It was a rare lapse, though -- and one to be expected at this stage of her life and career.

One reminder of just how young Gauff is: Most of the entrants in this year's junior Australian Open are older than she is.

Another: She is taking online classes and said she has been given permission to turn in homework late, "considering the circumstances."

Gauff is now the first American in 30 years to reach at least the third round in each of her first three major appearances.

Her next opponent will be No. 14 seed Sofia Kenin, a 21-year-old American who beat Zhang Shuai 7-5, 7-6 (7).

Osaka and Williams weren't the only former Australian Open champions to lose Friday. Caroline Wozniacki's farewell tour at the Australian Open is over. The 2018 champion lost her third-round match 7-5, 3-6, 7-5 to Ons Jabeur at Melbourne Arena.

Wozniacki said late last year that she would retire after the Australian Open to spend more time with her family and because she had been diagnosed with rheumatoid arthritis in 2018.

She sat in her courtside chair, wiping tears from her eyes before doing an on-court television interview.

"I think it was only fitting that my last match ended with a forehand error," the former No. 1-ranked Wozniacki told the crowd, smiling through the tears.

Jabeur is into the fourth round for the first time in a major and will next play Wang.

The Associated Press contributed to this report.