15-year-old Coco Gauff stuns title-holder Osaka at Australian Open

Unseeded 15-year-old Coco Gauff said she was in disbelief after beating reigning champion Naomi Osaka in straight sets in a major upset at the Australian Open on Friday.

The American, the youngest player in the draw and making her Melbourne debut this year, defeated the third seed 6-3, 6-4 to avenge a heavy defeat at last year's US Open.

Gauff plays compatriot and 14th seed Sofia Kenin or China's Zhang Shuai in the fourth round after a comprehensive victory, which underlines her rare talent.

Coco Gauff hits a return against Naomi Osaka during their women's singles match on Friday. AFP

This was the second instalment of a budding rivalry between Gauff and Osaka that could light up tennis for the next decade, having met at the US Open, where the 22-year-old Japanese won easily.

On that occasion it was the teenager who struggled with nerves, but the tables were turned this time.

Even so, the scale of the achievement had not fully set in for Gauff, the youngest woman to beat a top-five opponent since Jennifer Capriati defeated Gabriela Sabatini at the US Open in 1991.

"On the court I was definitely, what is my life? All these people clapping for me," she said.

"I'm still new to this, but I'm sure I'm not used to it." She was in such shock afterwards that "I don't even remember what I did, but I think I was more in disbelief so I didn't really know what to do." "I'm trying to think of what even happened today."

The sense of bewilderment took on a whole new level when Roger Federer became one of the first people to congratulate her.

"I was, like, 'Thank you,' trying to be calm. Inside, I was, like, Oh, wow!"

Osaka rattled

Gauff, who spent time training with Serena Williams in the off-season, had said she would be less nervous and more aggressive this time round in facing the world number four Osaka.

Naomi Osaka reacts during her third round loss to Coco Gauff at the Australian Open in Melbourne. AP

And so it proved as they rattled through the first five games in barely 15 minutes at the 15,000-capacity Rod Laver Arena.

Osaka fired into the net to drop serve in game eight and then racked up more unforced errors in gifting Gauff the first set in 32 minutes.

"Come on!" shouted Gauff, ranked 67 but fast making a name for herself as the most exciting prospect in women's tennis.

The young American carried the momentum into the first game of the second set, breaking a subdued and rattled Osaka, a two-time Grand Slam champion.

The Japanese broke back for 1-1 when Gauff volleyed into the net, but it was a rare mistake by the American and she broke once more, before serving for the match at 5-4.

The nerveless teenager clinched victory on the first match point when Osaka fired into the net.

Gauff announced herself to the sporting world at Wimbledon last year when, as a qualifier, she stunned Venus Williams in the first round.

She did the same thing to the seven-time Grand Slam champion on her Melbourne debut this week, then came back from a set and 3-0 down to defeat Sorana Cirstea on Wednesday.

When Gauff and Osaka met five months ago in New York, also in round three, then-world number one Osaka crushed the tearful and overawed teenager 6-3, 6-0 in a little over an hour.

"I learned a lot from that US Open. Not even just playing against Naomi, but the whole tournament," said Gauff.

"I think I was on edge and I had the three-setters in the first two rounds of the US Open, and I think that was just dealing with all of the media.

"I feel like now I'm more playing, just having fun.

"And winning is a cherry on top."

Agence France-Presse