Pironkova is struggling to close out this hold, she's lost four games in a row, but gets a helping hand as a Williams backhand catches the tape and leaps long.

Williams finds the net off the next point and makes a big hold.

Well played, Tsvetana Pironkova - the 32-year-old recovers to win successive points and edge back in front, but then is wrong-footed by a fierce Williams forehand winner.

Deuce.

Lovely stuff from Serena Williams, sizing up a slower second serve from Pironkova and reversing it into the space left behind bu the Bulgarian. 15-30, and the sniff of another break...

Image source, EPA

Backhand winner down the line from Pironkova, clever shot. That's deuce, and Williams is being made to graft for a spot in the semi-finals.

But the American closes out to consolidate her earlier break.

Tsvetana Pironkova won't give this up easily. She's still working hard to keep the ball in play and forces Serena Williams into a mistake. 30-30. The American has her opponent on the run again and this time Pironkova can only loft a forehand into the air, for Williams to swat away.

This time Williams makes no mistake!

She breaks in the opening game of the final set to take the lead in the US Open quarter-final.

Oh no!

Another superb rally, with both players being forced to cover almost every inch of the court.

Serena Williams finds herself sprinting back to retrieve a Pironkova lob, and the Bulgarian looks to have a straightforward volley at the net to hold serve... but she finds the gauze, and then sinks to the floor.

Suddenly, Williams has a third break point in this game.

That can't have been far away... almost on the money from Williams, but her return just creeps long and Pironkova staves off another break point. Deuce.

Pironkova dips low and flips a forehand long. Another chance for Serena Williams to break.

Oooh! That could have dropped either way, but Pironkova gets lucky as a speedy Williams forehand clips the tape, teases both players and eventually falls on the American's side.

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

Is Pironkova getting tight or tired? It feels like the latter to me. And that's when you start making the wrong decisions.

Make that a first break point of the set for Williams...

Big early chance here for the American.

Good response from Pironkova, finding some calm and composure to win three successive points and turn the opening service game of the final set back in here favour.

But Williams pounces on a short ball to ping down a backhand winner. Deuce.

Nervy start to the final set from the Bulgarian. Williams moves to 0-30.

Right then, who's taking this one!?

Tsvetana Pironkova returns from a bathroom break. And there's Jennifer Brady up in the stands, a potential final opponent for one of these players, if she can overcome Naomi Osaka...

Serena Williams has nipped off court too and heads back in wearing a face mask.

Pironkova will get the deciding set under way.

Serena Williams takes the second set with a sweeping ace that catches the T line and dashes away beyond Tsvetana Pironkova. It's her ninth ace of the set.

It's all square in the quarter-final.

Image source, Reuters

Serena Williams digs into her armoury and finds another ace to make it 30-30, and another to reach set point...

So Serena Williams will serve for the second set, as she looks to draw level.

She roars into a backhand winner and the yell echoes around the Arthur Ashe Stadium.

15-15.

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

This point was all about the will for both players. In the end Serena broke the spirit of Pironkova. Dejection for Pironkova with that drop shot at the end.