Emily Salley
BBC Sport at Wimbledon

A big roar as Novak Djokovic gets through another difficult service game, but there's little celebration from the 24-time Grand Slam champion as he tries to stretch out his back.

It wouldn't be surprising if we see the trainer called for at some point soon.

He just doesn't give up! Stretching, groaning, uncharacteristic errors - but Novak Djokovic holds serve once again with an ace.

Wu Yibing has only converted one of 11 break points.

Somewhat inevitably, Novak Djokovic finds a big first serve to save another break point.

Image source, Getty Images

Wu Yibing is going to get a fourth opportunity in this game after unleashing a vicious cross-court forehand winner.

Oh, that would have been sublime! Wu Yibing came flying across the court to recover a short ball but can only fire his cross-court reply wide.

Novak Djokovic saves the second break point with a line-kissing ace.

And then Wu has his head in his hands after sending a routine backhand long.

John McEnroe
Three-time Wimbledon singles champion on BBC Sport

Very rarely does Djokovic show a lack of discipline but he's got over anxious and I guess he felt like he had to go for more than he actually did. A very loose three points that could cost him here if he's not careful. He's going to be stressing.

Dear, oh dear. Back-to-back unforced errors off the forehand wing from Novak Djokovic. The first is particularly grim, burying the ball in the net on the approach.

With an undercooked attempted backhand drop shot... three break points.

Image source, Getty Images

Emily Salley
BBC Sport at Wimbledon

Even Novak Djokovic has to applaud that winner down the line from Wu Yibing, along with the rest of the crowd who bellow out Wu's name.

The Chinese player looks like he's got plenty of energy left to give, while Djokovic seems to be struggling a little physically, feeling the need to stretch in between points.

Sumptuous striking from Wu Yibing - a blistering down-the-line forehand for 40-0. Novak Djokovic, who didn't move, applauds on his strings.

John McEnroe
Three-time Wimbledon singles champion on BBC Sport

If Djokovic were to win this, he's got to play seven matches in 14 days and you don't want to get caught up in this one so early.

You've got to pace yourself and hopefully for him, get it done in four sets.

Two big first serves as Novak Djokovic escapes another iffy service game.

A very un-Djokovic shot on advantage, sending a down-the-line backhand several feet long.

Back to deuce we go!

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An ace to save the first break point.

A huge second serve down the T - 121mph - which Wu Yibing plants in the net to save the second.

Wu Yibing pounds a deep backhand return but it lands just beyond the baseline. "Wow!" exclaims Novak Djokovic. Was he surprised by the power? 15-all.

Wu continues to hit deep and forces a couple of forehand errors from Djokovic. Two break points.

There's lots of arm flapping from both sides of the net as Wu Yibing and Novak Djokovic exchange forehand unforced errors.

Djokovic makes lots of effort noises as he's locked in a baseline exchange and surprisingly, it's the Serb who blinks first. 40-30. "Wuuuuuuu" echoes around Centre Court.

Ace to hold.

Novak Djokovic comes through deuce without any additional drama. A scrappy hold.

He's doing plenty of stretching between points.

Image source, Getty Images

Wu Yibing is bringing the style, returning the ball behind his back after Novak Djokovic's serve is called out.

Either side of that, Djokovic hits aces to lead 30-0 but the Serbian soon has his arms outstretched in disbelief after mucking up a forehand.

More disbelief as he gives up 40-30 with another forehand error before Wu brings gasps out of the crowd with a drop-shot winner.

Wu Yibing holds to 15 at the start of the fourth set, finishing the game with a forehand volley on the approach.

What are we thinking - who's winning this? Thumbs up for Djokovic, thumbs down for Wu.

Image source, Getty Images

Emily Salley
BBC Sport at Wimbledon

Novak Djokovic is struggling with these lengthy baseline rallies. He's left doubled over after each one, huffing and puffing.

But he finds a way through eventually and strides back to his chair with a clenched fist. His son, Stefan, celebrates in the same way from Djokovic's box.