And that's it from us for this evening, but don't forget you can follow the men's quarter-final between Daniil Medvedev and Andrey Rublev on BBC Radio 5 Live Sports Extra by clicking the tab at the top of this page.

Elise Mertens also faces Victoria Azarenka later to complete the women's semi-final line-up, which will be played from midnight tomorrow.

And if you missed Serena Williams' enthralling win over Tsvetana Pironkova, then check out Saj Chowdhury's report here.

Catch you later...

One step closer to that 24th Grand Slam title for Serena Williams, who will face the winner of Victoria Azarenka and Elise Mertens in the semi-final tomorrow.

I'm with Serena, mums are absolute heroes.

And here is Serena Williams, on Pironkova's return after three years out: "It shows me how tough mums are! Whenever you can birth a baby, honestly you can do anything. We saw that today, she played unbelievable.

"I could barely win a match when I came back, so she's incredible. Her baby has to be younger than mine - you play a match, go home and you're still changing diapers, it's like a double life.

"I was feeling it a little in my legs and an hour in, for some reason, I got more energy. I can't do that if I want to keep winning.

"I play again tomorrow, I am used to playing back to back to back, but I need to figure out how t start a little bit faster.

"I keep fighting, I never give up. You've got to keep going."

Another zinger of a serve that Pironkova cannot return and Williams has three match points.

Ace to kick the game off from Serena Williams, followed by a wrist-rattling serve for 30-0.

This time the 23-time Grand Slam champions gets it!

Serena Williams takes a double break in the final set, and will serve for a place in the US Open semi-final.

Now it's Pironkova's turn to look on longingly as a backhand from deep drifts long.

Break point Williams.

Great line and length from Pironkova, or was it? Williams challenges a backhand that looked to have kissed the line, and HawkEye says it has. Deuce it is.

And now another chance to break. A super return backhand. Williams is looking to finish this off quickly.

Framer from Williams, high into the New York sky, but she follows up with two wielding winners to bring the Pironkova service game to deuce.

Russell Fuller
BBC tennis correspondent

It will be interesting to see how Serena holds out physically in a semi-final or a final. Will she have played just a little bit too much?

The first sign that Pironkova looks a little leggy, as she crouches down after a wayward forehand, wondering what she can do to stop Williams' momentum.

Straight back at it from the Bulgarian though, engineering a fiery backhand winner. 30-15.

Barry Cowan
Former British tennis player on BBC Radio 5 Live Sports Extra

It's different body language from Serena to the opening set - in the first set she almost took it on the chin but now she's got her opponent and she's not going to let go.

Pironkova finds the net off her backhand side, and Williams stings down an ace to hold.

Approaching two hours out on court in New York, but Serena Williams is showing no signs of fatigue - she slings a backhand winner past a stretching Pironkova for 30-15.

No messing from Pironkova either, rallying to 40-0.

Williams responds with an overhead smash on the high bounce from close range but can only drag her next return wide as the Bulgarian holds.

That's great from Williams, accelerating to the hold. She's clicking into gear.

Bit of sweat on the ball there? Williams nips off to find a towel and wipe it down, before rattling a thumping ace beyond Pironkova.

The Bulgarian seizes her chance on a slower second serve off the next point with a fierce crosscourt backhand winner. 15-15.