That breather - for the mind and body - seems to do Thiem some good as he crashes an ace down for 30-15 lead.

But that momentum doesn't last long. Tsitsipas levels at 30-30 and then soaks up the pressure in a baseline rally before springing forward to turn defence into attack.

That counter ends with him volleying in for a 40-30 lead and that means it is a break point...

Matthew Henry
BBC Sport at the O2 Arena

Here we go then. A one-set shootout for the biggest title of either players' career.

Image source, AFP

Here's a stat and a half... Dominic Thiem has won 88.9% of deciding sets this year. That's more than anyone else on tour. Tsitsipas? He's scrabbling around the 50% mark and about 50th place on the tour.

Clenched fists all around Camp Tsitsipas as Stefanos, dad/coach Aposotlos and other coach Patrick Mouratoglou exchange right-handed salutes.

Thiem decides he needs a breather - a tactical break, I'd suggest - and jogs off inside.

After a blip at the start with a rogue return, Tsitsipas gets back on track with a first serve down the middle and another ace out wide for 30-15.

A first set point arrives seconds later, Thiem saves that one, but can't do anything other than crack the second into the net.

All square!

Image source, AFP

Right then... Tsitsipas serving for the set.

Andrew Castle
BBC Sport tennis commentator at the O2 Arena

We're probably about five or 10 minutes away from the most important passage of play in this match. It seems like everything is getting ready for take-off in the third.

You won't catch Thiem doing a Kyrgios and checking out in this set. The Austrian wants to build rhythm and not simply save energy by tanking, rediscovering some of his mojo as an ace out wide moves him 40-15 ahead.

Another bruiser of a first serve - the speed gun clocks it at 139mph - rocks Tsitsipas. Game.

Matthew Henry
BBC Sport at the O2 Arena

The crowd feels stunned by this drop-off from Thiem. The spectators don't know what to make of it.

Thiem opens up a little on the Tsitsipas serve, well, he might as well. A loose forehand is a little too loose and flies long for 30-0, Tsitsipas crashing down an ace for 40-0.

Thiem, not looking comfortable receiving, smacks a forehand into the net on second serve and that's the game.

Tsitsipas is 12/12 on serve in this set! And he's one more away from levelling...

Thiem's coach Nicolas Massu - the man who has taken the Austrian to new heights this year - always has a bit of bassethound expression, but he looks particularly glum at the moment.

This gets him going though, a hold to love for his man.

He can't, can he? Tsitsipas still a double break up and looking good to take this into a decider.

Tim Henman
Former British number one on BBC Two

Credit where credit it due. Tsitsipas didn't get down on himself after losing that first set - he has taken advantage of a lapse in concentration of his opponent.

Matthew Henry
BBC Sport at the O2 Arena

One of the biggest cheers of the day for Woody Harrelson as the movie star appears on the big screen above the court.

Kingpin is in the house. Woody Harrelson, looking pretty sober, nods in appreciation at seeing Tsitsipas claim another strike. A hold for love for the Greek.

Those who went for a toilet break at the end of the first set might not see any of the second set at this rate...

Andrew Castle
BBC Sport tennis commentator at the O2 Arena

Isn't it amazing how a player can go to sleep. Thiem needs to be careful here - he could get rolled in this set.

Where is Dominic Thiem's head at? He's checked out. Tsitsipas is locked in.

After not seeing one break in the first set, two suddenly appear. London buses, etc.

Tsitsipas confidently steers a forehand winner down the line with Thiem stranded on the other side of the dancefloor.

Image source, pa

Excellent response from Stef the Greek. Granted he was given a helping hand by Thiem in the first game, but this is almost all down to him. Thumping winners, an ace and a lazy, long backhand from Thiem seals a hold to love.

Andrew Castle
BBC Sport tennis commentator at the O2 Arena

This is a lapse of concentration from Thiem. It happens a lot after the highs of a break.

Thiem rallies to save the first one when Tsitsipas skies a return and eventually it drops well outside of the lines.

The Greek takes his second chance, however, and that is the first break of the match.

Image source, EPA

After the euphoric high is the comedown... Dominic Thiem's head is still slightly clouded after the joy of clinching the first set tie-break.

Muddled thinking on his serve gives Tsitsipas three break points...