Naomi Osaka's shoulders are down. That backhand still isn't firing, another return drifting long, and all the momentum is with Coco Gauff.

Osaka stares at her box in frustration as she's undone by yet another brilliant Gauff serve out wide, but she's helped to 40-30 as Gauff double faults.

Now that's more like it from Osaka! Better movement, winning a tussle at the net, and we're at deuce.

Wow.

That is a horrible miss from Naomi Osaka, wide, loopy, long, and Coco Gauff breaks in the first game of the second set!

Hmm. It's Naomi Osaka's backhand that's letting her down, another one flying long over the baseline for 15-15.

Now then! Osaka's at the net, waiting to pat away a volley, but she snatches at it, sending it into the foot of the net, and it's break point to Gauff.

Daniela Hantuchova
Former world number five at Melbourne Park

Gauff looks like she thought 'of course I'm going to take the first set agaisnt the defending champion'. She just makes it look so natural. It looks like shes been thinking about this moment for a long time.

It's why she's going to be one of the greatest - if not THE greatest - of all time.

Is there another upset brewing...?

Naomi Osaka will serve to get the second set going.

After just 32 minutes, the 15-year-old Gauff has taken the first set over defending champion Osaka.

Superb, brave hitting from Gauff, and Osaka just could not blast her out of the rallies like she did in New York.

Gauff finished that set with eight unforced errors to Osaka's 14, and she's won 82% of points on her first serve.

A serve out wide - and Naomi Osaka sends her return out wide!

The first set goes to Coco Gauff. WOW.

Daniela Hantuchova
Former world number five at Melbourne Park

I don't understand why she tried to go up the line, it was low - it was the most difficult option. She's really confused out there now. She's panicking.

A superb first serve out wide from Coco Gauff, and Naomi Osaka can't return it.

Again, Osaka aims down the line and again, she misses, Gauff's box jumping to their feet as she races to a 30-0 lead.

Flat-footed from Osaka, missing right from the baseline, and three set points to Gauff.

What a start from Coco Gauff!

She'll serve for the first set.

Naomi Osaka misses her first serve...

Oh, and that's another miss on the backhand! Coco Gauff breaks at the first opportunity!

Naomi Osaka grins to herself as she goes for a smash out wide and misses, but she gets a 30-15 lead as Coco Gauff burns a challenge.

This is good from Gauff, constantly getting that extra ball back, forcing Osaka to move and play off her backhand, and it's 30-30 as a stretched Osaka nets.

Now then! Osaka misses an easy backhand and it's break point for Coco Gauff!

There's shoes squeaking all over the show as Naomi Osaka aims a forehand winner down the line and misses for 15-15.

Lovely from Coco Gauff, serving into Osaka's body, and Osaka whirls her backhand into the net in response.

Another miss on the forehand from Osaka is almost cancelled about by a Gauff double fault, but that's a superb, aggressive strike from Gauff to hold.

Novak Djokovic continued his title defence with a routine win over Japan's Yoshihito Nishioka.

The Serb needed just 85 minutes to rout Nishioka 6-3 6-2 6-2 and set up a meeting with Diego Schwartzman of Argentina.

Hungary's Martin Fuscovics awaits either Roger Federer or John Millman in the fourth round after beating American Tommy Paul 6-1 6-1 6-4.

Image source, Getty Images

Coco Gauff miscues a return of serve, which puts Naomi Osaka within touching distance of a hold.

And, after a superb rally, Gauff being pushed side to side and yet somehow getting ball after ball back, Osaka holds.

Coco Gauff gets the first point on the Naomi Osaka serve as she draws an error out of the Japanese backhand.

Lovely! A terrific net rally is won by Gauff, who lobs her way back into the point before smacking the ball emphatically home for a 0-30 lead.

Osaka's in a spot of bother but she gets on the board thanks to a few Gauff errors and wallops an ace down wide for game point, but that's long from Osaka, and we're at deuce.

Daniela Hantuchova
Former world number five at Melbourne Park

This is one of the best matches we could possibly ask for. Today has been a great day overall. The match between Serena (Williams) and Wang Qiang - it was the best woman's match we've seen so far in this tournament.

The conditions are just perfect for the players on court at the moment.

Lovely serving from Coco Gauff, going out wide and forcing Naomi Osaka to net on the backhand.

Backhand meets backhand for a shade before Osaka drags one wide and Gauff registers a love service hold of her own.

Coco Gauff floats a forehand long before she stands and watches a Naomi Osaka drive volley go sailing past her.

Wallop. Ace. Osaka holds to love.

David Law
BBC Radio 5 Live Sports Extra tennis commentator

Gauff is showing here that she's happy to unleash on the ball and I think that's a good sign. It's good because she's not going to go slow-ball and beat Osaka.

Two comfortable service games for Coco Gauff.