The sun is fully behind a cloud as Wozniacki nets to spark a Peng fist pump. 15-0, but the bouncing Wozniacki is scrapping, a backhand and two Peng errors putting her in charge. 15-40, two break points, two points from a tie-break....

Image source, EPA

Where did I put my upset claxon? Peng breaks at the last possible opportunity and will serve for the first set. The backhand volley isn't good enough from Wozniacki, putting the Chinese ahead.

The peach number that Wozniacki is wearing has a darker half that gives an unflattering illusion in the underarm region, if you know what I mean. Serving away from us as we watch, white visor to white visor, she is asked to defend defiantly for what seems like an age. In comes Peng, deft drop, Wozniacki not quite quick enough. 30-30. When the forehand yet agin lets her down, Wozniacki faces break point...

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Is it possible for the temperature to be cranked up? It's 28 degrees as it is. Either way, we're seeing some big ball-striking, a winner getting Peng ahead, with Wozniacki unable to control a forehand for 40-15. When Wozniacki drifts long once more, we're sent to overtime.

What a rally! Brutal, line-hunting stuff. 28 shots, the longest of the match so far. Ooohs and aaahs, Wozniacki defending until Peng nets. All that for 0-15...

Image source, Reuters

You wonder how many chances Peng will get, so wasting them isn't clever. Long with a forehand, twice, to allow Wozniacki back to pole position. Heavy breathing, a bounce or two of the ball, persuading Peng to go long once more. A hold by the skin of her teeth, Peng must serve to stay in the set.

That's not settled the 10th seed, though, who allows Peng back to 30-30 with her fourth double fault of the match. Ooofff, Wozniacki was for the taking there, but Peng dumped a double-hander into the net. This time the Chinese gets it right and the fist pump tells us we're at 40-40. From no sign of a break, we've had two and the Wozniacki serve is again being pressured. Pressure? Yet another double fault. Break point.

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...a ridiculous rally. Both players pushed way back, the ball at times lobbed over as if being patted around by a couple of club players. Wozniacki's patience is rewarded as Peng goes long, meaning we're all-square again. All together now..."it's only a break if you hold afterwards".

Peng behind looking to follow up the break in the biggest match of her life. Once more a cloud rolls over as Wozniacki plays her best forehand so far, hunting the corner and leaving Peng trailing. 0-30. Peng has Wozniacki stretching, but a trip to the tramlines gives gives Wozniacki two break-back points...

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Wozniacki is being challenged by Peng, who takes two long rallies, first with a backhand winner, then a loose Wozniacki backhand. Two break points. Peng again engages Wozniacki, who frailties on the forehand surface with a wild shank. We have a break, and it's gone to the underdog.

Jeff Tarango
Former professional tennis player on BBC Radio 5 live sports extra

"Both ladies are doing a good job of holding serve. We've seen a lot of breaks in other matches, in both men's and women's matches, but this set is very much commanded by serve."

That breeze to which my man Piers refers has the Stars and Stripes blowing with gusto high above the court. The sun, though, just dips behind a cloud as a pair of Peng errors allow Wozniacki back to 30-30. The Dane bounces, fiddles with the strings, but flashes a stare at her box when a forehand takes a trip to the net. A Peng backhand wraps it up. Still no sniff of a break point.

Piers Newbery
BBC Sport at Flushing Meadows

"The temperature gauge is sticking with 28C but it feels a lot hotter on Ashe right now, with the gusting breeze very welcome - for the spectators at least. Wozniacki's father, Piotr, surveys the scene from behind dark glasses, doubtless frustrated that there is no on-court coaching allowed at Slams. He is the master of that domain."

Image source, AP

Sweat glistens on the arms of Wozniacki as she gets the better of a baseline exchange. Polite applause, but much louder when Peng charges for a backhand and 30-30. They're getting stuck in to these rallies now, the ball getting the treatment from corner to corner. Wozniacki, though, lets a third double in for 40-40. What was I saying about the ball getting battered? Wozniacki smashes to Manhattan then leaves Peng static with a backhand to hold.

Jeff Tarango
Former professional tennis player on BBC Radio 5 live sports extra

"Wozniacki has also improved her first serve, her strength and her fitness. She is in much better shape than a year or two years ago. She has re-dedicated herself to tennis and has changed her racquet back to the one she had when she was world number one. I am proud of her for doing all that and coming through all the troubles she's had this year."

Peng is no stranger to the latter stages of the Slams, it's just being on her own that is a new concept. She holds the French Open doubles title and won Wimbledon last year. With the shadow of the umpire's chair creeping on to the blue rectangle that is the court, Wozniacki gets the better of a volley exchange before Peng has her on the run. A stunning backhand and a delightful drop get the job done to 15.

Jeff Tarango
Former professional tennis player on BBC Radio 5 live sports extra

"Wozniacki is very consistent and is doing a very good job of oscillating the ball from side to side. Peng can hit the ball so hard that Wozniacki doesn't want to set her up. It's a good tactic."