Wayward from Williams, she can't deal with the Bulgarian's unorthodox, slice-heavy style and loops a forehand off target. Pironkova breaks early in the second set.

And now Pironkova edges to a break point...

Ding! Serena Williams thuds down an ace that hits something in the distance at the back of the court and chimes like a cymbal.

But Pironkova isn't going away. The Bulgarian hustles to 30-30.

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

This set deserved to finish on a winner - and it was arguably her best shot of the match.

Brilliant. Pironkova dances around the ball to take it on her backhand, almost at ankle height, and slings a superb winner beyond a stunned Williams.

That is fantastic.

Image source, Reuters

No hangover from that double for Pironkova, who responds with a thumping ace.

Williams is unable to deal with another stinger, and the Bulgarian has two set points.

It's Tsvetana Pironkova, in her first US Open quarter-final, to serve for the opening set on Arthur Ashe. She edges to 15-0, but then double faults...

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

This has been a 9.5/10 set of tennis. It hasn't been one Serena Williams has played particularly badly. But this is classy from Pironkova.

Saved again!

Williams is showing some resilience here. More than six minutes into this service game, she flings down an ace to hold and force Pironkova to serve for the opening set.

Oh wow. What a wonderful lob that is. Pironkova leaves Williams stranded in the service boxes with a lovely disguise on her looping effort.

Set point number two...

Close from Tsvetana Pironkova, but not close enough - the 32-year-old challenges a backhand that looked long, and it was. Just. But Serena Williams is struggling to close this out, back to deuce.

Wonderful point from Serena Williams. Working her opponent around the court and stepping into a big forehand winner from mid-court to stave off that set point.

Pironkova flips a backhand deep, Williams can only find the tape in response.

Set point to the Bulgarian.

Serena Williams then, serving to stay in the first set.

A wide serve-forehand winner combo makes it 30-15 but the following backhand is loose and wide.

Was that the chance? Williams looks frustrated as Pironkova creeps to 40-30 and holds with a punchy serve. She's one game away from the opening set...

Ooh. Serena Williams steps forward to meet a high ball and lash down a forehand volley that nearly bursts Tsvetana Pironkova's strings.

She pushes the Bulgarian deep behind the baseline to make it 30-30.

Image source, epa

Great stuff! Serena Williams is forced to dig deep, but she anticipates the drop shot and deftly guides a cute pick-up backhand over the tape to leave Pironkova no option but to shoot wide.

A roaring forehand dusts off a love hold and it's greeted by a smattering of applause.

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

Does Serena have another level to go to? She might have to rely on Pironkova dropping her very high level of tennis.

Williams pounces on a slower second serve to take control of the rally, but somehow Pironkova slices her way out of trouble.

A deep forehand has Williams reaching, and her backhand it called out.

Pironkova holds to consolidate her earlier break.

Image source, EPA

Russell Fuller
BBC tennis correspondent

That was ruthless from Pironkova. Serena has certainly not been sleepy, she's been on it early on. But there are very, very encouraging signs for the nation of Bulgaria.