Deuce again as the world number eight looked to attack but went long.

Williams' radar is not working at the moment - a backhand finds the net, and she repeats the trick on 'advantage Pliskova'.

The American has been broken.

Williams saves as Pliskova fails with a backhand down the line.

She gets a second chance as Williams' shot clips the net and goes out.

Deuce as Williams goes long.

It's 32C out in New York, and no doubt a lot hotter down on the surface of the Arthur Ashe Stadium.

Double fault bring up the first break point.

David Law
BBC Radio 5 live sports extra commentator at Flushing Meadows

We are looking at the two best serves in women's tennis.

15-15.

What a pick-up on the half followed by a silky double-handed backhand winner from Pliskova.

15-30.

Ace and ace. Have some of that, says Williams.

40-30.

Jeff Tarango
Former professional tennis player at Flushing Meadows

Serena is pacing herself. Both players are just finding their way and not overexerting themselves. They have seen players come out and burn themselves out in the first set.

Williams returns into the net before Pliskova fires a forehand wide when she really should have landed it on the right side of the tramlines.

The Czech player eventually holds.

Cat Archer
BBC Radio 5 live sports extra on Arthur Ashe Stadium

Every evening, as the sun sets here at Flushing Meadows there is a spectacular hue to the Manhattan sky.

Here’s David Law bathing in it while commentating on Williams v Pliskova.

Image source, BBC Sport

Pliskova's 94mph serve is returned with 20% interest by Williams.

0-15.

The 26-year-old bites back with two good serves.

Pliskova reduces the deficit with a casual swipe with her forehand.

40-30.

Williams holds after her kicking-into-body second serve is not returned accurately by the Czech.

15-15 as Williams looks to attack at the net from her serve but finds the net.

The six-time winner delivers a fierce forehand which Pliskova stops running for.

30-15.

40-15.

Jeff Tarango
Former professional tennis player at Flushing Meadows

I would say Pliskova is favourite in this match but because Serena has spent so much time in Florida she is used to this humidity. That is a leveller.

*denotes next server

Williams has only dropped one set en route to this stage, and that was against Simona Halep-slayer Kaia Kanepi in the last round.

World number eight Pliskova, on the other hand, has yet to drop a set. She was particularly impressive against Australian Ashleigh Barty in the last round.

Does the Czech player also have the edge over Williams having beaten her on this stage two years ago?

Jonathan Jurejko
BBC Sport at Flushing Meadows

Karolina Pliskova doesn't have to dig too deep cognitively to find positive memories of beating Serena Williams.

The Czech eight seed can take confidence from knowing she beat Williams in straight sets here at Flushing Meadows when they last met - in the 2016 semi-finals.

Martina Navratilova says revenge will be a motivating factor for Serena.

"Maybe Serena underestimated Pliskova, who played very well, and maybe she was surprised by the pace coming off Pliskova’s racquet," she told BBC Radio 5 live.

"Pliskova can serve you off the court and she will have to try to do that today and keep the points short.

"But you know Serena is going to want to avenge that defeat."

If anyone is well placed to discuss how good Serena Williams' game is then it is Martina Navratilova.

The 18-time Grand Slam singles champion says Williams is answering some lingering questions from Wimbledon and looks "fantastic" in New York.

"She looked pretty good at Wimbledon but had an easier draw getting to the final so there was a lot of doubt," Martina told BBC Radio 5 live at Flushing Meadows.

"Still she got to the final but I think her lack of match play and lack of competition against top players showed against Angelique Kerber.

"Here she has had some tougher matches against tougher opponents and has been playing better and better. She looks extremely confident.

"I've been impressed by her footwork seems to be getting set for her shots and hitting well."

We are back at Arthur Ashe as Serena Williams continues her quest for her seventh US Open title.

Who stands in her way? Her 2016 nemesis Karolina Pliskova.

We have live text commentary coming up and audio commentary on BBC Radio 5 live sports extra for what promises to be a fascinating encounter.

Right. The action on Ashe is taking a break so we will too.

We'll be back when Serena Williams and Karolina Pliskova begin their quarter-final just after midnight in the first match of the night session.

See you shortly.

Here's the winner Juan Martin del Potro: "I'm so happy to reach another semi-final in my favourite tournament that is very special to me. Beating John in these kind of matches - it's epic. We were both tired in the end and I survived with my serves during the match.

On his 10-minute break between the third and fourth sets due to the extreme heat policy:

"During the 10 minutes I had a shower, re-taped my ankles and a lay down on table - I didn't want to come back again.

"The heat was strong, but this crowd and this energy..."

On his small group of hardcore supporters:

"If I meet my supporting group it might be dangerous! I'll stay in my hotel and let them walk around the streets. They've made the effort to come up from Argentina - flights, hotel and beers."

Del Potro will play Rafael Nadal or Dominic Thiem in the semi-finals.

Those two play later in the night session.

The men's semi-finals aren't until Friday so the former champion has time to recover from those fatigue issues.