Despite that spell at the top of the world rankings, Wozniacki has only ever reached one previous Grand Slam final, at this tournament in 2009 when she was beaten by Kim Clijsters. In the longest rally of the match so far, Wozniacki turns defence into attack and puts away a volley at the net, but the advancing Peng shows aggression of her own for 30-30. Danger for the Dane, averted with a remarkable squatting forehand (played from the crouching position of a wicketkeeper) and a kicking ace.

Image source, EPA

A quick change, Peng serving from the far end as we look on. In six previous meetings, Peng has beaten Wozniacki only once and allows the Dane back to 40-40 with a long forehand. A first sign of trouble, but negotiated with a deep backhand and a Wozniacki dump into the net.

Wozniacki, platted long blonde hair, belts down an early ace, but follows that with a sloppy double. The 10th seed, though, is looking confident, asking Peng to chase a forehand then having the Chinese fail with a return. Good hold.

The players have taken to the court and are going through that very curious tennis version of a warm-up - the knock-up. Wozniacki has shed a frilly pink tracksuit top to reveal a peach, while Peng is in a pink number of her own. There's still plenty of empty blue seats as the umpire calls time, with the Dane first to emerge from her chair. She'll have the balls first.

BBC Radio 5 live sports extra

And, while you're mulling that one over, you may want to supplement your enjoyment of this informative and entertaining live text with an audio accompaniment. You can listen to live commentary from Flushing Meadows on BBC Radio 5 live sports extra, here online, on digital TVs and via the BBC Sport and BBC Radio apps.

So, on another scorching Big Apple day, that is our immediate concern. Peng v Wozniacki for the right to meet either Williams or Makarova. The question is simple. Who do you think will reach the final? Are we set for a Willniacki demolition job, or will Peng and Makarova cause chaos in Queens? Text 81111, tweet using the hashtag #bbctennis,, external or get involved on the BBC Sport Facebook, external page.

If that question isn't floating your boat, ponder this. Who are sport's best friends? The inseparables? Woodbridge and Woodforde? Ballesteros and Olazabal? Redgrave and Pinsent? Kevin Pietersen and Andrew Strauss? You get my drift.

Piers Newbery
BBC Sport at Flushing Meadows

Image source, AP

"It's a very muggy 28C and another blowy day at Flushing Meadows, with the flags flapping furiously as Bruno Soares and Sania Mirza receive their trophy for winning the mixed doubles.

"The Arthur Ashe Stadium is an oasis of calm compared to yesterday evening when Roger Federer beat Gael Monfils in a match so exciting it even got a mention on the US sports news, albeit after lengthy discussion of last night's opening NFL match (the Seahawks won).

"Caroline Wozniacki and Serena Williams have been making the headlines on the women's side of the US Open, with the good friends favoured to make it through to the final today."

In a tournament of upsets, Peng has proved to be most adept at making the big girls cry. Agnieszka Radwanska (seeded four), Roberta Vinci (28) and Lucie Safarova (14) have all fallen to the 28-year-old world number 39, who has never won at Tour level.

Wozniacki has had her share of tests too, not least a three-set stonker against Maria Sharapova in the last 16. The bottom line, though, is that the former world number one, now Roryless, is through to her first Grand Slam semi for three years.

Peng Shuai and Ekaterina Makarova (whose friendship status is unknown) will have a thing or two to say about that potential final love-in, though.

It is they who stand in the respective paths, with unseeded Chinese Peng, in her first Grand Slam semi, up first against Wozniacki.

See, Oscar's words could be very relevant to the 2014 US Open. When Caroline Wozniacki was down in the dumps following her split from Rory McIlroy, she was cheered up by her BFF Serena Williams.

That was a Miami beach in June, this is the last four of a New York Grand Slam in September, where they are each one win away from a final meeting - sympathising with a friend's success and all that.

Image source, AP

"Anybody can sympathise with the sufferings of a friend, but it requires a very fine nature to sympathise with a friend's success."

I wonder if Oscar Wilde liked tennis.