Serena Williams beats Karolina Pliskova to reach semi-finals
American 17th seed wins 6-4 6-3 at Flushing Meadows
Argentina's Juan Martin del Potro through to semi-finals on Friday
Third seed beats John Isner 6-7 (5-7) 6-3 7-6 (7-4) 6-2
2017 champion Sloane Stephens knocked out by Anastasija Sevastova
Latvia's Sevastova through to semi-finals with 6-2 6-3 victory
Rafael Nadal plays Dominic Thiem in the last match of day nine
Extreme heat policy in place as New York temperature causes problems
*Del Potro 3-3 Isner
Isner pummels another double-handed backhander down the line.
30-0.
Then Del Potro produces the best shot of the match so far with a 102mph forehand howitzer, on the run, from the back of the court.
Stunning.
30-15.
Del Potro 3-3 Isner*
The 6ft 10in American then rockets down a double-handed backhand winner down the line.
He has the power.
30-15.
40-15.
Del Potro then gets right over the ball for a beautiful whipped forehand winner.
It's pure silk.
Del Potro 2-3 Isner*
15-0 and then Del Potro smacks down his third ace of the match - the same number as Isner has managed so far.
30-0.
*Del Potro 2-3 Isner
Del Potro has a 7-4 lead in head-to heads.
But Isner has a 40-0 lead in this service game.
The American world number 11 holds serve as Del Potro goes long.
Del Potro 2-2 Isner*
The Argentine world number three serves an ace to bring up 30-0, before Isner returns long from another belter of a serve.
We have a rally!! Which Del Potro wins with a booming forehand.
Held.
*Del Potro 1-2 Isner
Big John Isner, whose wife is expecting their first child soon after the US Open, has a nice little fan next to him.
Have any of the other players had one next to their chairs?
Delpo to serve.
#bbctennis
John McEnerney: DelPo to out muscle Isner will those booming forehands even with the dodgy wrists. Even though JI is playing in front of his own people DelPo will have everyone else & maybe a few yanks he’s such a likable guy who has had plenty of setbacks over the years! Vamos DelPo
*Del Potro 1-2 Isner
30-0.
Make that 40-0 as Del Potro makes another error from the Isner serve.
Isner serves an ace down the middle to hold to love.
Del Potro 1-1 Isner*
Two big serves from Delpo gets him out of a spot of bother.
It feels like there's a break coming soon.
Del Potro 0-1 Isner*
Delpo inches to the net and takes the first point of his service game as Isner loops a return out of play.
The American levels with a great return.
15-15.
30-15.
Great work by Isner who attacks the net and beats down backhand volley winner.
30s each.
*Del Potro 0-1 Isner
It helps if you have a 136mph serve in your arsenal though.
Advantage Isner.
The American then serves an ace down the T.
Held.
*denotes next server
*Del Potro 0-0 Isner
Isner goes long from the back.
He really didn't want deuce on his opening service game.
*Del Potro 0-0 Isner
0-30! On the Isner serve in the first game!
Delpo finds the net with the next point before Isner shows he's not just a big server with soft-hands at the net which delivers a winning drop-shot.
30-30.
40-30 courtesy of a 124mph second serve.
*Del Potro 0-0 Isner
Those who like to keep tabs of players' birthdays will be delighted to know that it's Del Potro's 30th on 23 September.
The US Open title would be a lovely gift wouldn't it?
Play is under way.
Del Potro v Isner
Image source, Getty ImagesDelpo has looked great so far at this tournament and has yet to drop a set.
Isner, on the other hand, has played two five-setters.
What energy reserves does the American have?
It's knock-up time at the moment - play will start in moments.
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Del Potro v Isner
If this doesn't last four hours-plus we'll all be surprised.
John Isner will be hoping not to make a US one-two on Arthur Ashe today by defeating Argentina's world number three.
Isner, 33, is in good Grand Slam form this year having reached the Wimbledon semis. A match we all remember..er.. fondly.
Del Potro won this title back in 2009 and reached the semis last year only to be beaten by defending champion Rafael Nadal.
Who do you back in this encounter folks?
Stephens 2-6 3-6 Sevastova
Here's our US Open semi-finalist Anastasija Sevastova:
"I think it was very physical today and tough to play. It was so hot in the stadium. I just kept fighting.
On Stephens' recent win over her in Montreal: "I didn't play that good in Montreal and she played amazing. i knew it'd be different today. I took my chances.
About the hot conditions: "Grandstand is quite open, but you don't feel it much. Here the sun was on one side and the shade on another. I was glad I was in the shade in the last few games."
Whether she will watch her possible semi-final opponents, Serena Williams or Karolina Pliskova, later:
"I'll watch tonight's match. I like watching night matches."
Jill Craybas
Former world number 39 on BBC Radio 5 live sports extra
Stephens wasn't moving as well as she usually does. A lot of that had to do with Sevastova. She defended so well.
Stephens 2-6 3-6 Sevastova
And... Stephens' shot hits the net.
Game.
Set.
and
Match.
Sevastova.
The defending champion is out.