There's a grimace with almost every Djokovic serve at the moment.

And the odd limp afterwards.

You suspect more treatment may be needed once this first set is over.

Felix Auger-Aliassime is sniffing blood here a bit, some big hits forcing Djokovic to move when he doesn't want to. Deuce.

Emily Salley
BBC Sport at Wimbledon

The crowd are trying their best to show Novak Djokovic their support.

They don't want this match to come to an early end because of injury.

Tim Henman
Former British number one on BBC Two

You could tell what that meant to Felix Auger-Aliassime just by his reaction.

He was down 15-40 and won four points in a row to show some more resilience in this first set.

The calf problem is particularly bothering Djokovic when he serves - both propelling himself into the air and then on landing.

Unreal ace from Felix Auger-Aliassime. And then another!

Superb response from 15-40 down to hold.

Image source, Getty Images

Beware the injured tennis player?

Novak Djokovic puts a smash away - at a second attempt - to bring up two set points...

The first one goes begging via a backhand that flies just long. And a big serve helps Auger-Aliassime save the second.

Tim Henman
Former British number one on BBC Two

Novak Djokovic still isn't looking totally comfortable.

He is flexing and stretching that calf...

A big cheer erupts as Novak Djokovic emerges from the changeover.

Of course, he's actually just a break of serve away from winning the first set.

And errant forehands like the one Felix Auger-Aliassime just ballooned out of court will help...

Felix Auger-Aliassime is keeping himself warm while Novak Djokovic is being treated.

There's a bit of a worried low hush around Centre Court - the crowd will have been really looking forward to this quarter-final.

Todd Woodbridge
Former Wimbledon doubles champion on BBC Two

Novak Djokovic gave a big reaction to whatever the problem is when he hit that backhand down the line at 0-15. He gave a bit of an 'Oooh'.

From the way he is stretching, it looks like it could be the calf or the Achilles or potentially the plantar fascia area of the foot itself.

They are testing the stability of the ankle into the calf at the minute, which isn't a good injury. It is one you tend to get at the backend of your career rather than at any other point.

The trainer is massaging Novak Djokovic's lower left calf.

The Serb looks a bit bothered by this.

It's now officially a medical time-out.

Image source, Getty Images

Tim Henman
Former British number one on BBC Two

It looks like Novak Djokovic was stretching his calf or his Achilles between points there.

There was definitely a bit of grimacing going on as he shook out that left leg.

Djokovic carries on for now.

And the leg issue doesn't bother him too much as he holds serve to love - sealed with an ace down the T.

But the trainer is coming on.

Novak Djokovic is tenderly assessing a problem with his left leg.

Seems OK for now but one to keep an eye on.

Is he calling for the trainer?

Image source, PA Media

Emily Salley
BBC Sport at Wimbledon

There's one guy in the crowd, who's dressed in a red t-shirt and sat behind the umpire's chair, that is whipping up support for Novak Djokovic.

He waits for a moment of silence in between points before getting to his feet and belting out "let's go Novak, let's go". The rest of the crowd follow his lead.

Big hold from Felix Auger-Aliassime there, aided by some big serves from break point down.

That was the first real test of either's player mettle.

Marion Bartoli
Former Wimbledon champion on BBC Radio 5 Sports Extra

The amount of pressure that Novak [Djokovic] is building here, if Felix [Auger-Aliassime] doesn't hit the perfect serve, Djokovic will get the point.

We're still at deuce in this key eighth game of the first set...

Tim Henman
Former British number one on BBC Two

Novak Djokovic is recognising that he has to go hard into the backhand of Felix Auger-Aliassime at any opportunity. That's his opponent's vulnerability.

Auger-Aliassime's first serve percentage has also dropped to 43% now.

Image source, Getty Images

We've not had too many lengthy rallies but Felix Auger-Aliassime gets the better of one such instance.

However, a couple of errors creep in from the Canadian from 30-0 to deuce. The first sniff of a chance of a break of serve.

Yep, and Djokovic outlasts a man 14 years his junior in a cross-court rally to give himself a break point... but a big serve-volley from Auger-Aliassime puts paid to that.

Todd Woodbridge
Former Wimbledon doubles champion on BBC Two

Novak Djokovic has played over 25,000 points in the men's singles at the Wimbledon championships but he has only won 55% of them, yet he has claimed seven titles.