American teenager Coco Gauff has turned the tables on Naomi Osaka, knocking the defending champion out of the Australian Open with a straight sets victory on Rod Laver Arena.

The 15-year-old Gauff moved through to the fourth round with a 6-3, 6-4 win over the third seed and world number four in just one hour seven minutes.

The teenager — who was ranked 685th in the world in January 2019 — hit the headlines midway through last year, when she made it to the round of 16 at Wimbledon, along the way beating seven-time Grand Slam singles winner Venus Williams Magdalena Rybarikova and Polona Hercog before losing to the ,eventual champion, Simona Halep.

The pair met at last year's US Open, where Osaka swept Gauff aside 6-3, 6-0 on Arthur Ashe Stadium.

This time, however, it was a different story, as Gauff held her nerve on the big stage to outplay Osaka and clinch a spot in the round of 16.

Coco Gauff clinched the biggest win of her career in just over an hour on Rod Laver Arena. ( AP: Lee Jin-man )

"Two years ago I lost in the first round in juniors. Now I'm here. This is crazy," she told Channel Nine after the match.

"I was just telling myself one point at a time and keep fighting. You never what happens on this court."

The American was asked when she was going to get back to homework, replying: "Probably tomorrow because I want to go to sleep tonight".

"But my teachers, they're giving me some time because, considering the circumstances, they're letting me submit some of the assignments late. So that's OK."

Gauff makes an impressive start

Osaka started aggressively, perhaps trying to reproduce her easy win at the US Open. She came out hitting the ball hard, ripping a brilliant forehand winner in the second point of the match.

But Gauff appeared calm and survived a double fault to hold her opening serve.

Osaka had a straightforward hold for 1-1, then Gauff matched her as she displayed no lack of confidence on the big stage.

Osaka held to love, capped by an ace down the "T" — but again the American answered with a similarly easy hold.

The champion was having to work harder than expected. In the sixth game, Gauff got to 0-30 on Osaka's serve, after a great point where she forced her opponent backwards, the Japanese star threw up a lob and the 15-year-old put away the overhead with aplomb.

She couldn't make it count, however, as Osaka came back to hold serve, but it was already clear that this was a much more competitive outing than at Flushing Meadows.

Naomi Osaka could not find the answers against an impressive Coco Gauff at the Australian Open. ( AP: Lee Jin-man )

Osaka was making a lot of unforced (and forced) errors, and after Gauff held for 4-3, the big chance finally emerged in the eighth game.

Osaka had led 30-15, but two straight errors gave Gauff the first break point of the match. Sighting a second serve, the American held in the rally before her cross-court shot stayed low and Osaka netted the return to hand over the break.

The defending champion was desperate to stay in the set, but she kept making errors, and Gauff held to love to take the first set 6-3 after 32 minutes.

The variety of Gauff's play was impressive, as she combined hard groundstrokes with good movement — and a first-serve percentage of more than 80 — plus the occasional moonball to break up Osaka's rhythm.

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Gauff maintained her momentum early in the second set, breaking Osaka in the opening game to take a big hold on the match — but two double faults in the following game helped hand the break straight back.

Osaka immediately held to love to ramp up the pressure on the teenager, but Gauff refused to take a backward step and levelled at 2-2.

Two games later, Gauff served her first ace — a solid 185 kilometre per hour effort down the "T" — on the way to a love hold at 3-3.

The American was playing solid tennis and not getting overawed by the occasion. In the seventh game she raised the pressure on Osaka, who made two errors to slip behind 0-30.

The third seed then had an opening to hit a winner down the line, but as she came forward Osaka hit a nervous shot into the net to bring up three break points.

She saved one, but Gauff then hit a deep approach and Osaka netted again for the vital break.

The momentum was all with the American — she held to love, and though Osaka won the next game to stay in the tournament, Gauff took advantage of more errors from her opponent to close out a brilliant performance.

She will now play the winner of American 14th seed Sofia Kenin and China's Shuai Zhang for a place in the quarter-finals.