What an exchange!

Enthralling, exhausting - 26 shots, the longest rally of the match - but it's Serena Williams who comes out on top as a tired Pironkova forehand finds the net.

Williams breaks.

Smart save from Pironkova on the first, approaching the net and guiding a volley into space.

Two more break points for Williams.

Long from Pironkova, the Bulgarian who recently returned fro a three-year hiatus, and the 32-year-old is then outfoxed by a prodded one-handed Serena Williams backhand.

Suddenly she's 0-40 down, and Williams has three break points.

Pironkova edges to 15-30 on the Williams serve and must sense another break point opportunity, but that is quickly extinguished by Williams as she claws back to 40-30.

Pironkova is undone by a slower, leaping second serve from Williams and cracks a backhand into the tape as the American holds.

Ouchy. Williams skids into a forehand and lands on the floor behind the baseline. She's straight back up on to her feet and looks to be moving OK.

Pironkova packs some serious punch in her serve and jabs her way to 40-15, before whipping a superb, low-bouncing backhand winner beyond Williams to hold.

Speedy hold from Serena Williams. The first one of those we've seen on either side for a while.

Alexander Zverev watches on in the stands, enjoying some lunch by the looks of it.

Image source, EPA

Just long from Williams. And Pironkova takes a big hold!

Saved by Tsvetana Pironkova, who follows up with a volleyed winner at the net - a great change of direction on the shot there from the Bulgarian. Great hands.

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

I think Serena has started to get into Pironkova's head now. Serena is showing no emotion.

WIlliams capitalises on the second serve and places a fiery forehand into an unreturnable spot.

Advantage, and break point to the America.

A mini fist pump from Serena Williams as Pironkova jabs a baseline stroke into the net.

Deuce.

This quarter-final hits the hour mark, it's been an entertaining contest so far.

Williams launches a crosscourt backhand that Pironkova can only direct into the tramlines.

30-30.

Finally Williams closes out as a Pironkova backhand drifts into the tramlines.

A big hold. It's back on serve in the second.

Image source, epa

Williams nudges her way to advantage but Pironkova sizes up a forehand and thunders it beyond the six-time champ for another deuce.

Williams' 10th ace hands her advantage once more, but the Bulgarian is playing some incredible tennis - a sizzling backhand down the line to level up.

Tsvetana Pironkova challenges a Williams ace, but it just kissed the line at 104mph. Williams leaps into a forehand on the next point but punches it wide.

And then goes long... she's let Pironkova back into this one. Deuce.

Fizzing forehand winner that leaves Pironkova swiping at thin air. Williams is finding her range.

30-0.

Got it! Is this the start of the Serena revival? The 23-time Grand Slam makes Pironkova play and she can only dig a forehand into the tape. Williams breaks back immediately.

Great depth from Serena Williams and it forces the Bulgarian into the net.

Break back point for the 38-year-old American...

Image source, Reuters

Some real fury in a forehand winner from Serena Williams and she stares down Pironkova afterwards. 15-30, but a huge backhand rattles the gauze and loops out.

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

Serena has been knocked off balance for large parts of the match and in that game it was by her own hesitation.