Juan Martin del Potro will not want this already delayed match to go on. The winner will have to play Rafael Nadal tomorrow, one of the biggest tests in this sport.

The Argentine breaks back.

Image source, Reuters

Kerber is halfway there.

The errors were the big difference. Daria Kasatkina has all the shots but made too many mistakes. Kerber meanwhile just does not miss.

Jelena Ostapenko could have wrapped the first set up by now. She has put Dominika Cibulkova's serve under immense pressure with some superb returns. But she can't hold her own serve. Each player has two breaks each.

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Groans from the Centre Crowd crowd. Two double faults from Daria Kasatkina and all that good work in the last game is wasted. She hands Kerber the break.

The Russian doesn't hide her displeasure, waving her racquet around and muttering at the back of the court.

Image source, AFP

Saj Chowdhury
BBC Sport at Wimbledon

It’s a bromance that began in March and continues to blossom.

Milos Raonic was given a workout on the practice courts by 2001 champ Goran Ivanisevic.

The Canadian was wearing what looked like knee braces but it could have been to keep his legs warm.

His Croatian coach - and former big server - is the right man to have in your court with John ‘Ace’ Isner up next.

Boom! Daria Kasatkina has turned up in this Wimbledon quarter-final.

She unleashes a forehand at the end of a great rally which flies away for a winner and then gets out the big fist pump to celebrate.

Kasatkina only looks like a teenager but she's proving she's up for the fight on Centre Court.

Suddenly Daria Kasatkina is dictating the points. She plays another of those leaping backhands, hit with both feet in the air like you sometimes see from Gael Monfils.

Here are two break back points...

Kim Clijsters
Four-time Grand Slam champion

Dominika Cibulkova would have been seeded, had it not been for the decision by the All England Club to give a seeding to Serena Williams despite her world ranking of 181.

"It has motivated her. I saw her play in the third round and she was very fired up. She plays her best tennis when she has that fighting spirit behind her. I understand for her it was very disappointing. I agree that Serena Williams should be seeded but of course she is going to be upset. Maybe we should introduce a rule so everyone knows it is going to happen before it does."

She's just won three games in a row and has the lead in the first set.

Image source, Reuters

Craig: Jack Nicholson with John McEnroe and Nicholson introducing him: "Heerrees Johnny". And then to Mac: "You can't handle the truth!"

Halcali: Federer and me

Well played, Daria Kasatkina. She saves one break point with a good serve and then on the second she plays a lovely drop shot. The Russian may be just 21 but she is like a throw back with all her different spins and slices.

She jumps into a powerful backhand to take the game. Centre Court is impressed.

Tracy Austin
Two-time Grand Slam champion on BBC TV

What a start from Kerber. She's smothering Kasatkina, hitting with accuracy and really not letting her get set.

Image source, Reuters

Kerber bends low hitting a trademark forehand while almost sitting on the grass.

Two more break points...

A double fault from del Potro at the end of a shaky service game and it is Simon who has edged ahead in this fourth set.

We could be in for a long match on court two.

Click here to watch the match

Angelique Kerber's coach Wim Fissette, formerly the coach of Johanna Konta, is nodding in approval in the German's box. His player has started well. She holds comfortably.

Billie Jean looks to be giving out a free coaching lesson in the Royal Box. She shadows a backhand from her seat.

Image source, Reuters

Tracy Austin
Two-time Grand Slam champion on BBC TV

Kasatkina starting to feel the rhythm. Crucial for Kerber to get the service hold and not give too much confidence to Kasatkina, make her feel comfortable.

Sonia Oxley
BBC Sport at Wimbledon

Four-time Grand Slam champion Kim Clijsters is gearing up for an invitation doubles match later this afternoon, but there is only one match she is feeling nervous about and it isn't a tennis match:

“The Belgium match later,” she said. “I was in the pub the other day and they were all singing “It’s coming home!” and I was like “It’s coming to Belgium!”

Her match is down as “not before 17:00” on Court 12, but she is definitely going to be hoping it’s over before 19:00 BST.

Daria Kasatkina whacks her racquet with her palm after hitting another of her sliced backhands into the tape.

That's better from the Russian. She works away at the Kerber defence with her forehand and then wins the point at the net.

She sweeps her wispy brown hair out of her eyes before banging down a big first serve to hold for the first time.

John McEnroe
Three-time Wimbledon singles champion on BBC TV

Kasatkina has frozen out there. She is not moving, just reaching for balls.

Credit to Kerber, using her experience controlling the court and dictating, Kasatkina has to hit herself in the thigh with her racquet a couple of times, get herself going.

Image source, Getty Images

Mark Petchey
BBC Sport tennis commentator

I enjoy watching the early skirmishes in matches like this which have been disrupted to see if there are new tactics being used.

Will Del Potro continue to pepper the forehand side from his serve?