A love service hold for Norrie, who has really given Zverev a great deal to think about.

At a set and a break up, Zverev may well have thought he would have seen off the Briton by now, but Norrie, aiming to reach the second Grand Slam quarter-final of his career, is fighting hard in this one.

Heather Watson and Joe Salisbury are making the most of their alternate slot in the mixed doubles.

They have just taken the first set off eighth seeds Ellen Perez and Jean-Julien Rojer and they have just broken at the start of the second set.

We are at the two-and-a-half-hour mark for this match and it's been a very good watch.

Zverev's backhand is really firing up and he gets the fourth game of the fourth set off to a fine start with a superb winner. A fifth double fault from Zverev takes it to 40-30, but he produces an early forehand and sweeps it on to the line for a fine winner.

Jonathan Jurejko
BBC Sport in Melbourne

Cam Norrie's popularity in Melbourne will also have boosted by having an Aussie in his camp.

The British number one sought out the help of Stephen Huss - the Australian who won the Wimbledon doubles title in 2005 - before the start of this season.

No wonder volleying has been introduced to Norrie's game.

Before the tournament, Norrie said Huss wouldn't be working with his team at Melbourne Park and would largely be sticking to training weeks.

But it looks like Huss is sat next to Norrie's long-time coach Facu Lugones today - unless he has a doppelganger.

This is a match of the highest quality at the moment. Both men are having to produce some wonderful tennis, with neither player giving anything up.

Zverev was just warming up nicely, but Norrie does excellently to see off the threat and get a hard-fought hold. Still on serve in the fourth set, with Zverev two sets to one up.

Remember, we have live radio commentary on Radio 5 Sports Extra, so press the play button to listen along.

Image source, Getty Images

An excellent hold from the sixth seed, from 15-40 down. Zverev is having to match Norrie's aggression with some excellent attacking play of his own.

Cameron Norrie is chasing down everything, but Zverev is able to see off those two break points.

David Law
Commentator on BBC Radio 5 Sports Extra

I think Zverev is far short of his best compared to where Norrie is at his levels. Zverev is the better player overall, having reached Grand Slam finals and won Masters titles.

But he is getting his head scrambled by the British player.

Lobbing a 6ft 6in opponent is not the easiest of tasks, but Norrie is able to do so after a drop shot to mix it up. Zverev slashes the next shot wide and Norrie, again on the attack, finds himself with two break points.

Mark Woodforde
17-time Grand Slam doubles champion on BBC Radio 5 Sports Extra and BBC Sounds

Seventeen unforced errors off the forehand from Zverev.

If Cam had that information on the change of ends, he would probably be heading into that side.

A solid hold to 15 from Norrie, who responds with a clenched fist to start us off in the fourth set.

Zverev is the reigning Olympic champion, but yet to win a Grand Slam title.

He was two sets up in the 2020 US Open final before losing a five-set thriller to Dominic Thiem.

Zverev has also lost in the semi-finals at Grand Slams on five occasions - the 2020 Australian Open, at each of the past three French Opens and also the 2021 US Open.

Is this his year?

Jonathan Jurejko
BBC Sport in Melbourne

Zverev's visceral roar tells us how much stress he has been feeling today.

Now the sixth seed has regained the lead it is hard to see Norrie fight back once again.

The Brit has popped off to the loo, perhaps getting a splash of cold water on the face too, as he tries to sharpen body and mind for another push.

Image source, Getty Images

Cameron Norrie has never beaten Alexander Zverev and will need to go the distance if he is to do so today.

An excellent response from Zverev after losing four games in a row at the end of the second set and the German is one set away from reaching the quarter-finals.

What a point. Cameron Norrie saves the third set point, producing a brilliant lob when he needed to after some superb defensive work.

But Zverev then forces set point number four.

Norrie saves the first set point as Zverev goes long on the forehand. The second set point also goes too thanks to Norrie's excellent drop shot.

Zverev with one more chance though.

Zverev with three set points after forcefully sending away a forehand winner at the net.

Norrie gets a bit lucky as a shot bounces off the net twice to fall on Zverev's side and that sees the Briton hold.

Zverev is now serving for the set though.

David Law
Commentator on BBC Radio 5 Sports Extra

Zverev is managing the match well. I don’t think there is anything to choose between them, especially in the passages of play.

The Zverev serve is doing enough damage to keep him ahead and he’s taken advantage of a bit of a dip from Norrie at the start of this set.

Zverev moves one game away from the third set. Norrie had done excellently to win a point with a stretching volley at the net after been dashing around the court, but he can't build on that as he angles a forehand long and Zverev holds.