Oooh, double fault from the Austrian.

That's 40-30. Does Zverev sense an opening? Nope, he lofts a forehand long from deep.

Thiem holds.

Dominic Thiem rattles into his service game with a knowing nod after each delivery.

Alexander Zverev lets out an uncomfortable groan and can only stick frame on a wide ball.

40-15.

Jonathan Jurejko
BBC Sport at Roland Garros

The cavalry arrive. That third game was played out to the background noise of excited chatter from those waiting to enter Chatrier at the change of ends.

It prompted a few 'sssshes' from those already inside, although didn't distract the players or attract the attention of the umpire.

The umpire does politely tell them to get a shift on and find their seats though as the players get ready to resume.

How about this for a stat...

If Alexander Zverev wins in five sets today, he will become the first player in the Open Era to win four consecutive five-set matches at a Grand Slam.

Endurance.

Punchy from Zverev, he holds and it remains on serve in this quarter-final.

Four games gone on in the opening set of the day on Court Suzanne-Lenglen and we are still on serve.

Yulia Putintseva is the only unseeded player left in the women's singles competition, with both her and opponent Madison Keys aiming to reach the final four at Roland Garros for the first times in their careers.

Millimetre-perfect from Thiem, powering a backhand on to the whites of the Roland Garros lines to leave a full-stretch Zverev swinging at thin Parisian air.

A touch of serve and volley from the German makes it 40-15.

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

It might be something to do with the weather but there are a lot of empty seats for a match of this status featuring two of the brightest hopes of this generation.

Oh, hello!

That's the best of the bunch so far. Zverev forced to backpedal, before slamming a forehand between his legs while facing the wrong way.

Thiem coaxes a deft volley into the space left behind from the net. The leggy Zverev just can't reach it. Thiem holds.

We're seeing some long rallies in the opening exchanges. That'll no doubt suit Dominic Thiem, who has spent nearly three hours less on court than Zverev so far in this tournament.

The Austrian piles in a forehand winner to make it 30-0.

Over on Court Suzanne-Lenglen, world number 98 Yulia Putintseva of Kazakhstan takes on America's 13th seed Madison Keys in the first of the quarter-finals in the women's singles.

Putintseva, 23, is playing in only her second Grand Slam quarter-final after also making the last eight at Roland Garros in 2016 when she lost in three sets to Serena Williams.

Despite the difference in ranking, Putintseva has already shown her class by knocking out Great Britain's Johanna Konta, the 22nd seed, in the opening round and Barbora Strycova of the Czech Republic, the 26th seed, in the last 16.

For Keys, also 23, this is her best run in the French Open, although she has played in a Grand Slam final before when she was beaten by fellow American Sloane Stephens in the 2017 US Open final.

Jonathan Jurejko
BBC Sport at Roland Garros

This is a Roland Garros quarter-final between two of the sport's most exciting talents.

The French fans appear to have forgotten that fact. Chatrier is barely half full.

Image source, BBC Sport

First double fault from Zverev brings up deuce.

Thiem pats a return into the tramlines and then sends a backhand long as the German closes out.

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

A bit of cat and mouse in these early stages as they work their way into the match.

It's such a big occasion for both of these players so it's normal to see a little bit of nerves at the start.

That ball is being blasted down at Dominic Thiem at serious height from the opposite service line.

Alexander Zverev whips his way to 30-0 before the Austrian manages to engage his serve, only to find Zverev lumbering towards the net for a volley into the tape.

30-30

Away we go then, Alexander Zverev to serve first.

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

Alexander Zverev plays close to the baseline, has one of the best two-handed backhands in the game, a good serve because he's 6ft 6in and is getting more comfortable coming into the net.

Dominic Thiem has big take-backs and takes big swings and on a clay court he has the time to play his shots. He sits more at the back of the court and puts more spin and slice on the ball and gets the ball to kick.

Image source, Reuters

Dominic Thiem leads opponent Alexander Zverev 4-2 in their head-to-head stakes, despite being seeded five places lower than the German.

The Austrian 24-year-old is looking to make a third successive semi-finals at Roland Garros, having never made it beyond the fourth round of any other major.

Three of Thiem's four victories over Zverev have come on clay, including the 2016 French Open third round, although he was beaten by the 21-year-old in Madrid last month having earlier knocked out Nadal.

A win today would see him draw level with Zverev as the player with the most number of Tour wins so far this year.

Image source, Getty Images

With Roger Federer opting to sit this tournament out, Dominic Thiem and Alexander Zverev are two of the remaining eight at the French Open best placed to challenge world number one Rafael Nadal.

The winner faces a potential semi-final against Novak Djokovic, with Rafa lurking on the other side of the draw.

"I'm top three in the world now," said Zverev. "Other young guys are coming up strong - in the end we'll be very high soon.

"With time we will see who can become the next Grand Slam champion, who can become the next world number one.

"Right now Roger and Rafa are dominating, but we are doing everything we can to change that."

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

Zverev has shown incredible staying power this week. Some people have questioned his level of play but at the end of the day he won.

Playing so many five setters leave question marks about his fitness but he's a machine and works so hard on his physical conditioning with Jez Green, the former conditioning coach of Andy Murray.

He is in extremely good shape so I don't think it it will play as big a role as some people think.

This match should be an absolute blockbuster.