Live Commentary: Serena Williams vs. Roberta Vinci - as it happened

Hello, and welcome to Sports Mole's live coverage of the US Open semi-final between Serena Williams and Roberta Vinci.

We have already witnessed one surprise today. Will we see another? We will find out over the next couple of hours. Serena Williams is bidding for the Calendar Grand Slam and Roberta Vinci is looking to join her Italian compatriot in the final. There are sure to be a few nerves out there on Arthur Ashe Stadium.

As you would expect, Serena has been the centre of attention for the past two weeks and at times, it has looked like she has struggled to handle the pressure. She was fortunate to get through a couple of her early matches against Kiki Bertens and Bethanie Mattek-Sands, but with the finishing line getting closer, she has really upped her game. That's going to have to continue against Vinci, but this match shouldn't be treated as a foregone conclusion. Serena is human just like the rest of the players on the WTA Tour and this could be the biggest 28 or 29 hours of her career.

As for Vinci, she is effectively free-rolling in this match. She has lost all four of her previous meetings with Williams and has never reached the last four of a Grand Slam. That said, Pennetta's win will have only increased her motivation to make the final. An all-Italian final would be huge for their nation and if she recognises that the pressure is on her opponent rather than herself, she is more than capable of stunning the world. Not just the tennis world. The sporting world.

Anyhow, we are ready to get going in New York. It will be Serena, who walks to her mark to a warm reception, who will serve first.

Serena opens with a couple of aces to move herself to two game points before Vinci concedes the opener on the first with a return into the net.

Good start from Vinci, who forces her opponent into an error at the net for 30-0, but the American strikes back to get to 30-30. Williams is sensing a break point but credit to Vinci, who holds after her opponent sends a forehand long.

BREAK! Vinci is creating openings for herself on the Williams serve, but so far, she isn't taking them. A glaring miss with her forehand helps Williams create two game points. However, from then onwards, Serena looks nervous, and despite saving a break point, she goes long on a second and Vinci has the break without doing a whole lot. Interesting.

BREAK! After the opening two points are shared, Serena crunches a couple of returns back at her opponent and she has two opportunities to break back. She then lands a third, this time on the stretch, and she is understandably pumped.

We are only in the fifth game and Serena is already shouting "COME ON!" after every point. To her credit, she puts together a quite ruthless game, unleashing a number of forehands and drive volleys to seal a love hold.

BREAK! The pressure is all on Vinci at this stage and Serena is aware of it. She's on the attack at the start of each point and it pays off, with a return off a second serve proving too hot to handle for the Italian.

Vinci gets just her third winner of the match on the board, but that is as good as it gets. Williams is in full control and a hold to 15 takes her to within one game of the first set.

SET! I'm starting to feel a little sorry for Vinci. Her serve looks so weak, probably because Williams is returning it with so much ferocity. The 21-time Grand Slam winner misses out with two set points, but on the third, Vinci goes long with a backhand when attempting to sneak into the net. Business as usual as far as Williams is concerned.

When you have just been dominated for the last 20 minutes of a set, what is the best way to respond? By breaking your opponent's serve at the start of a set. It looks as though Vinci will do just that when a cross-court return winner brings up three break points, but the response from Williams is unreal. A cross-court backhand winner saves the third of three break points before an ace, her fifth, registering 122mph helps complete the game. She looks supreme.

Vinci ended the last game with plenty of head shakes, but to her credit, she breezes to 40-15 in next to no time. Another onslaught from Williams sees the game extended to deuce but the American makes a couple of unforced errors and Vinci is on the board for the second set.

Williams is barely breaking sweat, but that is what happens when you are drilling down 120mph aces. Number seven and eight are recorded in this game and she holds to love. I'm not too sure what Vinci can do to get back into his contest.

There's so much pressure on Vinci to make her first serve because she knows what is coming if she doesn't. Vinci creates three game points, but the problems start when she sees the line. Williams attacks and gets back to 40-30, but she dumps a forehand into the net to allow her opponent to restore parity. Good hold.

BREAK! Maybe I spoke too soon. Vinci catches the line with a return to move to 0-30, before doing the same, this time with more pop, on the next point to create another three break points. Williams, somewhat inevitably, saves the first with an ace, as she does with the second. Unreal. Can she save the third? She can't! A backhand goes wide and the Italian breaks!

We are a witnessing the trademark Williams lull right now. All of a sudden, she looks completely de-energised and frustrated. She takes the opening point with a forehand winner but the rest of the game belongs to Vinci.

Serena starts the next game with two uncompromising serves before Vinci floats a second-serve return long of the baseline. Bad miss. The game is sealed when Vinci does exactly the same on the first game point. Better from Williams though.

At 15-15, Vinci produces one of her biggest serves of the match to force Serena to go long. Where has that been all match? However, Williams turns thing around to bring up a break point, but the American squanders it, miscuing a relatively easy forehand. Vinci holds with some aggressive play of her own and she is one away from taking this match into a decider. I didn't think I would be writing that.

When the points are short, Serena is winning almost every point, but when Vinci is given the chance to mix up the speed of the ball, Williams is all over the place. She comes through with a hold to 15 but Vinci may be on to something here.

SET! Williams begins a must-win game with a cross-court forehand winner, but she misses a gimme of a backhand - it's an absolute shocker. It's 15-15, but Williams strikes back with a return winner. She proceeds to over-hit a backhand to move the game along to 30-30, but an error from Vinci gives her a chance to break back. Vinci finds the one-two though - it's superb. Williams then nets a return and it's set point to Vinci, who takes it by getting Williams on the run who proceeds to go long. We have a game on here!

That's the first set that Vinci has ever taken off Williams. It has taken her 10 attempts. If she takes another, she will reach her first Grand Slam final.

At 15-15, Williams gets lucky with a net cord to get to 30-15, before Vinci nets a return off a well-direct first serve. The game is sealed with a serve down the middle and she is back on track.

BREAK! Vinci takes the opening point, but a double fault and a winner from Williams sees the American reach 15-30. The world number one then steps into a second-serve return and Vinci has no answer. It's two break points to Williams, but she misses out on the first, dumping a return into the net. Should have made it. She converts the second, though, quite brilliantly. A tense point ends with Serena drilling a drive volley behind her opponent and she has the break.

BREAK! Williams looks in the zone again, and a 122mph takes her to 30-15. Both players tamely find the net to see the score progress to game point for Williams, but she is picked off by Vinci after making her way to the net. The Italian is pumped and she steals the break back when Serena throws in a double fault. This match is incredibly tense.

Williams concedes the first point after placing a volley into the top of the net before Vinci steps into court to put away a smash. Another mistake from Williams gifts Vinci three game points and the American remains alive with a forehand winner. It's the Italian's game though, after a net cord sends a shot from Williams into the tramlines.

This is some match now. Vinci starts with a crunching winner but Serena responds in kind, and is going absolutely berserk. That only intensifies when she catches the line with the backhand of the match before she lets out a shriek when Vinci can't return a first serve.

Vinci looks on course for a comfortable hold to love but her second double fault of the match helps Williams stay active. Not for long though, with Williams sending a return into the net.

BREAK! My word. After taking the opening point, Williams double faults...TWICE IN A ROW. Vinci should have two break points but she misses a backhand volley at the net. Such a let off for Williams. The American makes it to game point, but it's her turn to make a blunder on a backhand volley. Williams gets a second game point with an ace registering 126mph but Vinci's response is INSANE! She completes a 20-shot rally with the deftest of volleys and puts her hand to her ear to address the New York crowd. Brave! What a point though. Vinci has the momentum and she breaks through when Williams goes long with a forehand!

Indeed. She's had to reapply strapping to her ankle or foot too.

Serena goes a break down in deciding set, stadium starting blaring out 'Let It Go' #harsh — Sian Cowper (@siancowper) September 11, 2015

The first two points are shared, but Williams steals the advantage with a smash into the corner. A second smash takes the American to two break-back points, but the first goes begging, as does the second as Williams slips when attempting a forehand. Another shot into the net gives Vinci a game point, but she double faults. The tension! The second serves of Vinci are slowing down but she survives. She is one game away from one of the biggest shocks since...ever.

Vinci moves three points away when her aggression forces an error out of Williams, but it's such a gutsy response from the defending champion, who converts a smash after several are defended by her opponent. Two big serves move Serena to 40-15, before a second smash winner seals the game. What a game we have coming up. Can Serena save the Calendar Grand Slam?

MATCH! Williams nets a second-serve return to once again leave Vinci three points away, and make that two with a super drop volley on the bounce. Wow. It's 30-0, and make that 40-0! Serena nets a volley and it's three match points to Vinci! She only needs one - another drop volley seals the most incredible of wins. IN. CRED. I. BLE.

Am I going too far by saying that is one of the biggest shocks in Grand Slam history? In terms of the occasion and their previous head to heads, I'd say so. Roberta Vinci has just fought back from a set down to beat Serena Williams, who had won her last 33 Grand Slam matches in a row. Vinci is understandably emotional and speechless.

We have an all-Italian final, ladies and gents, but of course, you predicted that before the start, right? Tomorrow evening, she will face Flavia Pennetta in the US Open final. There's going to be plenty of pasta being eaten over the next 24 hours!

That will be all from us tonight, folks, but we'll see you tomorrow night. The weather might play its part but fingers crossed we get the match completed. Goodbye for now.