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Oof! An exquisite a backhand winner down the line eases Djokovic to 30-0 and he's quickly 40-0 up thanks to a razor-sharp ace. A minor blip allows Muller to steal a point, but another ace seals the deal and Djokovic seems in a ruthless mood.

A confident opening by the 31-year-old, picking up the opening two points with a couple of thunderbolt serves. At 30-0 down, the Djokovic return begins to hum, the Serb pinging a dreamy forehand winner down the line to halve the deficit.

Sloppy from Muller, slapping a forehand into the tape and the underdog could be in trouble. Ah, but bravo! An ace to the wings. 40-30. Muller with his nose in front once again and he comes through his first test with a crisp forehand winner.

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Even those with fame and fortune dream of being a rock star and based on this picture Novak Djokovic could put on a bit of a show were he to decide to play at Wembley. The Serb pretended to play guitar with a hologram of country singer Keith Urban at the Melbourne Park. It's one way to spend your free time, I guess.

The warm-up is nearly over which means the match is about to start. Strap yourselves in.

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Is the man for Luxembourg a dark horse, a player we (not Djokovic, of course) should take more seriously? The world number 42 has sent two seeds packing already at Melbourne, beating Roberto Baustista Agut and John Isner. Could Djokovic be the 31-year-old's next scalp? No sniggering and guffawing, please.

Hello! Aimee here, wishing you a Happy Australia Day and bringing you live text coverage of Novak Djokovic's test against Gilles Muller. Once the Australia Day pyrotechnics are over we will have some tennis to talk about. Until then, why not gaze at a picture of some fireworks.

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That's all from me, folks. As ever, thanks for your company. I'll leave you in the very capable hands of Aimee Lewis, who will take you through top seed Novak Djokovic's fourth round tie against Gilles Muller. Until next time, it's good morning from me.

The last time Serena and Venus met, it was Venus who came out on top in three sets in Montreal. Should they both make it to the semi-finals, it will be their 25th meeting, with Serena leading the head-to-head 14-11.

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Asked what her inspiration was for that victory, Venus Williams says: "Definitely my sister Serena. She's the ultimate champion, and my fans who have stuck by me through thick and thin. The first set wasn't easy, it was 6-3 but it was about an hour and in the second set she started even better. In the third set I went in a trance and just wanted to win."

Williams raises her arms aloft, jumping her way to the net and shooting a smile to her box. It's a huge win for the 34-year-old, who will now play fellow American Madison Keys in the next round. If she comes through that, she could face sister Serena in the semi-finals.

There are no hints of nerves as Williams edges to match point and then seals the match with a booming ace down the middle. The American reaches the quarter-finals of the Australian Open. It's her first last eight appearance in a Grand Slam since 2010. The 18th seed beats the sixth seed, nine years her junior.

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Martina Navratilova has had little to applaud in the last 20 minutes as she's seen her charge Radwanska suffer under a barrage of big hitting from Williams.

The American looks to have a new lease of life in this set, chasing balls down at will and reading the Radwanska serve really well.

Three break points for Williams but she only needs the one as Radwanska hits wide. Williams to serve for the match.

Williams mixes it up and throws in a forehand slice that just oozes spin and Radwanska fails to deal with it. The American is just brimming with confidence now as she moves to 40-15 on serve.

But a forehand volley drifts long - there's been a few of those today - and a backhand glides long to haul this game back to deuce. Williams then sprays a third ball long and she's suddenly facing break point.

Unfazed, Williams just keeps on battering that ball - and this time, it stays in. Back to deuce. A couple of game points go wanting, but Williams clings on to consolidate the break.

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Three break points for Williams! What a turnaround. The American's forehand is in scintillating form and Radwanska just can't do anything about it. The Pole finds herself scrapping along the baseline once more and eventually breaks down, gifting Williams the break to love. It's sensational stuff from Williams right now. Just sensational. But can she keep this up?

What a battle we have on Rod Laver Arena, Williams is giving it everything out there while Radwanska is scrambling around and making the American fight for every point. It's a close game, but Williams seals it with a 116mph serve - the fastest of the match.

The crowd are on their feet as Williams picks up a drop shot, leaps one way to pick up a volley and then stretches the other to pick up another for a winner! Sensational! Break point is her reward and she takes it with a crisp forehand volley. Williams is not done yet. We're back level, folks.

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Williams comes out fighting at the start of this set, racing to a 30-0 lead. But it's agony as she spoons a smash way out of court, narrowly missing a line judge. Suddenly, she finds herself at 30-30. And two points later, she concedes the break. How important will that mistake on the smash prove to be?

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It's evident that Williams' isn't hitting the ball as hard as she was in the first set and the American is also a touch slower to the ball. Radwanska takes advantage, clinching the game to level the match and take us into a decider.

Good hold from Williams, who stops the run of five straight games going against her, but has it simply delayed the inevitable? Radwanska will serve for the set shortly.

Williams is in serious bother here, with Radwanska stretching the American from side to side and even throwing in the odd cheeky drop shot. Williams tries coming to the net in an attempt to mix it up, but she is promptly passed. An ace later and the game is done.

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Radwanska is looking super confident right now, the Pole whipping up a lob that Williams doesn't even bother looking at, the American knowing she's been beaten.

At game point, a Williams forehand is called out. She ponders a challenge but decides against it. We then have a shot of sister Serena telling her to challenge and then raising her eyes in disbelief when she sees Venus decide against it.

Williams unravels, losing the next two points and it's another break for Radwanska. We're heading for a deciding set, folks.