Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

What a tie-break that was. That is really impressive. Dimitrov had to withstand serious serves from Kyrgios in that set.

Mishit from Grigor Dimitrov. A little framey, and that allows Nick Kyrgios a chance.

No. Dimitrov has three set points, he comes up with the goods on the next point...

And a double fault from Kyrgios. That's the set to Dimitrov.

Image source, EPA

Russell Fuller
BBC tennis correspondent at Melbourne Park

Grigor Dimitrov has played an exemplary tie-break so far.

Kyrgios riffles a forehand winner down the line. There was some feeling in that.

The Australian goes long off a deep Dimitrov return from the next point. The third seed leads 5-2.

Another superb rally, but another that Dimitrov gets the better of - Kyrgios looked a little static there, using his wingspan rather than his feet to reach the shot.

The Australian finds the net. That's 4-1 Dimitrov.

Great movement from Dimitrov, drifting into the net and pouncing to volley a backhand winner.

Kyrgios pulls it back to 2-1 to the Bulgarian.

Grigor Dimitrov follows up a big serve with a punchy forehand to go 1-0 up in the tie-break.

Image source, EPA

A big ace just when he needed it for Nick Kyrgios.

The ball is coming off that racquet at some speed, the Australian grunts his way into a powerful forehand that Dimitrov can't return to follow and gets the Hawk Eye call on the next point.

That just kissed the line - 40-0. Ace - game.

Piers Newbery
BBC Sport at Melbourne Park

"One text message! It's all you had to do!"

As ever, Kyrgios is as gripping a watch when sitting at the changeover as he is out on court. It seems somebody forgot to mark up his racquets, and there are a few ashen faces in his player box. It's going to be a tricky debrief later.

Longest rally of the match so far I reckon, 21 shots... but it is Dimitrov who comes out on top after Kyrgios pats a sliding backhand into the guts of the net.

His return is long from the next serve. Dimitrov holds.

Brilliant from Dimitrov, scuttling to his right and powering a forehand winner across court and beyond Kyrgios.

A rare double fault brings it back to 40-30, and a scintillating Kyrgios winner that almost curls around the net and just inside the line takes us to deuce - fantastic!

Nick Kyrgios is chirping away at his box again. Something about "figuring it out"?

He goes 30-0 down on Grigor Dimitrov's serve, before lofting a backhand beyond the reach of the despairing Bulgarian.

"I like it, aha aha," go the crowd...

Image source, AFP

A point for Grigor Dimitrov. They're like gold dust against the serve at the minute.

That is until Nick Kyrgios double faults. A way in for the Bulgarian?

Nope. Another big serve from Kyrgios is met with a backhand return that drops long.

Piers Newbery
BBC Sport at Melbourne Park

"I've no idea which one's a 54 and which one's a 55 now!" Kyrgios bellows towards his box at the changeover. "Which one is a 54? And which one is a 55?"

Dimitrov glances over quizzically, and the Kyrgios box shake their head and look bemused in unison. It's anyone's guess...

Business end of the first set. Grigor Dimitrov has come closest to making the break, but Nick Kyrgios has been impressive throughout.

He serves to stay in the set, forcing Dimitrov wide and the Bulgarian's return finds the bottom of the net.

Ace, ace, boom, boom.

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

That is some of the best serving you will see at any point on tour this year. It was astonishing.

It's battle of the serves at the minute.

Anything you can do... Dimitrov responds to that explosive Kyrgios service game with a couple of belters of his own.

The Australian sneaks a point to make it 30-15, but that's all the Bulgarian is prepared to give up.

Piers Newbery
BBC Sport at Melbourne Park

After years of Henmania and Murraymania, British tennis watchers will be able to relate to the attention heaped upon Kyrgios by the Australian media and public. It's 42 years since Mark Edmonson became the last Aussie male to win the title in Melbourne, and Kyrgios follows the likes of Pat Cash, Pat Rafter and Lleyton Hewitt as the latest great hope.

It feels like the sentiment towards him is changing here - "why the nation is falling back in love with Nick Kyrgios", as one newspaper put it this morning. There is no doubt that winning matches and looking like a serious title contender has helped.

And when there is a 'Countdown to Kyrgios' clock on the TV, and the fact that he went for a haircut on his day off made the news bulletins above the latest from President Trump, you know the nation is watching.

Nick Kyrgios, black compression sleeve copping the length of his right arm, cracks a huge serve that beats Dimitrov with the new balls.

He follows up with another ace, and another. Zing! Well, that's a service game done and dusted in under a minute...

Image source, Reuters

Leon Smith
GB Davis Cup captain on BBC Radio 5 live sports extra

I am absolutely buzzing for Kyle. It is such a big moment for him. We have seen that every match he has gone through has meant something else to him. He didn't panic at all when he lost the first set and kept his composure and I am so pleased for him.

I don't think it was physical (losing the first set), just nerves. It was a massive occasion for him. What was really good to see was Kyle wasn't playing best tennis but his serve kept him in it in the first set.

It is another big step forward for him.