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Now: women's semi-final - Serena Williams beaten 1-6 6-3 6-3 by Victoria Azarenka
Naomi Osaka beat Jennifer Brady 7-6 (7-1) 3-6 6-3 in earlier semi-final
Winners will meet in Saturday's final at Flushing Meadows
S Williams 6-1 3-4 Azarenka*
The clock ticks past the hour mark... this is the most exhausting hour of tennis I've watched in a long time.
The second was probably Osaka v Brady earlier.
The intensity of both matches has been incredible. Simply amazing!
S Williams 6-1 3-4 Azarenka*
Williams has got to figure this one out, quick sharp. Azarenka is matching her intensity now and reaping the rewards.
Serena lets out a roar which will be heard outside Ashe, even though the roof makes it airtight, when Azarenka goes long to give her a narrow advantage.
Two break points come Azarenka's way, but they disappear down the drain as Williams mixes up her first serves to get back on track.
*S Williams 6-1 2-4 Azarenka
The adrenaline is pumping through Azarenka's veins now. She bounces back out from her chair and whacks down three hefty first serves on her way to a blink-and-you'll-miss-it hold.
Williams huffs on the baseline. She's no idea where this has come from either.
S Williams 6-1 2-3 *Azarenka
David Law
BBC Radio 5 Live Sports Extra tennis commentator
You never know we could be all square in 15 minutes. It would have been an unfair way for Victoria Azarenka to go out in straight sets after the way she has played in this fortnight
S Williams 6-1 2-3 Azarenka*
Wowzers. Azarenka is back in the match! She crunches another backhand winner to grab that break point, twirling on the spot like Daniel LaRusso in celebration as she heads back to her chair for a breather.
*S Williams 6-1 2-2 Azarenka
Image source, GettyRight then... momentum with Azarenka here. A 30-0 lead gives her a glimpse of a break on the horizon, then it stares her square in the eye after the Belarusian sticks away a cross-court backhand winner.
Now then...
*S Williams 6-1 2-2 Azarenka
Woah there! A chink of light for Azarenka. Williams makes an UNFORCED ERROR (caps necessary for emphasis tonight) with a wayward forehand before a serve out wide can only be dumped into the net.
A hold to love for Azarenka.
S Williams 6-1 2-1 Azarenka*
Blimey. This is stunning stuff from Williams. This is why she is the GOAT.
S Williams 6-1 2-1 Azarenka*
Image source, GettyHalf a glimpse for Azarenka at 15-30 before the door is slammed firmly shut.
A 102mph ace out wide puts Williams within a point of the hold and the American great seals the deal with another monster forehand winner to finish off a 15-shot rally.
*S Williams 6-1 1-1 Azarenka
Could that be a pivotal moment? Doubt it, but trying to give hope to the Vika fans among you.
*S Williams 6-1 1-1 Azarenka
This is brutal... another break point for Williams.
Azarenka's coach tries to remain zen, but his stomach must be churning.
The Belarusian digs herself out of that hole though, scrambling to deuce then whacking winners of either flank for a much-needed hold.
S Williams 6-1 1-0 Azarenka*
In the time it took the buffering of my feed to sort itself out, Williams was already off the mark in the second.
So I can't tell you too much other than it was a rapid hold. Two aces and a service winner.
S Williams 6-1 Azarenka
There is a deluge in New York. And I'm not just talking about the heavy rain outside Ashe.
Williams whacked 12 winners in that 34-minute opener.
S Williams 6-1 Azarenka
David Law
BBC Radio 5 Live Sports Extra tennis commentator
Victoria Azarenka will be disappointed but I don't think that there was anything she could have done in that first set. Serena Williams crushed the ball from the very first rally and rocked her back on her heels.
S Williams 6-1 Azarenka
Another double fault from Azarenka. Oh dear. Vika stands on the baseline, holding her nose with her thumb and index finger.
The international sign of 'I'm trying not to go crackers here'.
That helps bring up set point numero dos which Williams takes with another one-two crackerjack.
First blow out wide, following it up with a sock straight down the middle.
S Williams 5-1 Azarenka*
Image source, GettySorry Vika, spoke too soon. Azarenka loses her way with a double fault and that helps Williams, who is hammering every return with the force of a paintball gun.
Set point... Williams can't get near this one though as Azarenka bazookers a bakchand winner down the line to stay alive. For now...
S Williams 5-1 Azarenka*
Azarenka looks hurt by how she has started this match. You can see it in her eyes. This will make her feel better though... she races 40-0 ahead.
S Williams 5-1 Azarenka
Russell Fuller
BBC tennis correspondent
Presuming that the first set is already lost, Victoria Azarenka will need to start very quickly in the second to stay in this match
S Williams 5-1 Azarenka*
Normal service resumes, though, Williams landing some heavy first serves to get over the line at the second time of asking.
One game away from the opener....
*S Williams 4-1 Azarenka
Hang on... Azarenka suddenly finds her gear, knocking an inch-perfect lob over Williams' head, then stepping into put away a volley and then it's deuce.