Federer beats Youzhny 6-1 6-7 (3-7) 4-6 6-4 6-2
Rogers and Gavrilova play longest women's match in US Open history
World number one Pliskova beats Gibbs 2-6 6-3 6-4
Seeds Dimitrov and Berdych knocked out, Del Potro through
Day four of US Open at Flushing Meadows
Russell Fuller
BBC tennis correspondent at Flushing Meadows
Watching Federer now is how I felt watching him on Tuesday night. He clearly is struggling. Even if he does win today, he'll have played nine or 10 sets going into the third round.
Federer 6-1 6-7 (3-7) 2-3 Youzhny*
I'll be completely honest, I didn't see this coming an hour ago. Federer's gone right off the boil.
And the strain on that back...
Federer 6-1 6-7 (3-7) 2-3 Youzhny*
Advantage Youzhny as another Federer forehand from the middle of the court goes begging.
Federer saves it with a body serve and waves his fist towards the crowd, but there's a groan around Arthur Ashe as his backhand finds the net to hand Youzhny break point number three.
And he takes it! A fine forehand gets cracked beyond an advancing Federer, and there's a genuine gasp from the crowd.
Image source, Getty Images
Leon Smith
Great Britain Davis Cup captain on BBC Radio 5 live sports extra at Flushing Meadows
Wow. This is interesting now. Youzhny senses something is not right. He is making things very difficult for Roger Federer.
Federer 6-1 6-7 (3-7) 2-2 Youzhny*
An ace gets Federer out of a mild bit of bother but that's a terrible response to a lob, Federer going for the overhead and spanking it into the net instead.
And now break point comes up as Federer makes a real meal of a forehand! 36 unforced errors now! He saves the break though as Youzhny nets, and that's deuce.
*Federer 6-1 6-7 (3-7) 2-2 Youzhny
Youzhny's backhand is starting to properly fire now. He's hitting it flat down the line, and it's thrown Federer off.
If he could wipe out the double faults then it'd be easier to keep that pressure on. Mind you, Federer's helping him along, tamely floating a return into the net. And again! Game, Youzhny.
Cast your minds back 20 years...
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Federer 6-1 6-7 (3-7) 2-1 Youzhny*
That was the first time Federer lost a set on a hard court against Youzhny since the Stockholm Open in 2000.
A speedy service game from Federer as he holds to love for the first time since midway through the first set.
Leon Smith
Great Britain Davis Cup captain on BBC Radio 5 live sports extra at Flushing Meadows
I thought it was a normal dip at the start of the second set but that looked more than that.
It gave Youzhny hope and he tested Federer's movement more. He started targeting his forehand corner. Roger didn't respond that well. He didn't look like he was moving as freely.
I don't know if it is because we are looking for these things but he didn't look as sharp. He wasn't twisting and turning and defending in the same way.
*Federer 6-1 6-7 (3-7) 1-1 Youzhny
Federer scratches his ear, adjusts his headband and tonks a loopy backhand wide. Youzhny has just found some extra speed in these past few games.
The Russian stays in a rally that Federer is dictating but Federer slaps a forehand flat past him, but the Swiss nets the next delivery for 40-30.
Down comes the ace from Youzhny. All square.
Federer 6-1 6-7 (3-7) 1-0 Youzhny*
Federer serves to get the third set under way, and there's a little "come on!" from the Swiss as Youzhny sends a return into the net. He'll hold.
*Thiem 3-2 Fritz
Image source, Getty ImagesOver on Louis Armstrong, sixth seed Dominic Thiem has broken serve in the first set against American Taylor Fritz.
On Grandstand, 18th seed Gael Monfils has taken the first set 6-4 against another American, Donald Young.
News isn't as positive for ninth seed David Goffin. The Belgian has lost the first set against Argentine Guido Pella 6-3.
Federer 6-1 6-7 (3-7) Youzhny
This is only the fifth set Youzhny has won against Federer. The world number three just looked to have switched off there when he had the chance to serve out the set.
Federer 6-1 6-7 (3-7) Youzhny
Three chances for Youzhny...
And that's tame from Roger! Into the net. Who saw this coming?
Mikhail Youzhny takes the second set.
*Federer 6-1 6-6 (3-6) Youzhny
A sterling backhand from Youzhny, curling the ball beyond Federer with such speed, puts him two points to the good.
And three set points come whizzing up as the Federer forehand goes sailing merrily long.
Federer 6-1 6-6 (3-4) Youzhny*
But now Federer is on the defensive after a beautiful backhand down the line from Youzhny. The Russian comes to the net and puts it away. He's back ahead with a mini-break.
*Federer 6-1 6-6 (3-3) Youzhny
Can Fed keep his serve this time?
The back looks fine there! A smash from the back of the court and it's past Youzhny in a flash.
They change ends all-square.
*Federer 6-1 6-6 (3-2) Youzhny
Youzhny goes down the line, finding a really nice length, and Federer sends his backhand low into the net.
The Russian challenges again as a forehand return is called out, and the technology, after a brief pause while someone tries to figure out how to work it, agrees with the line judge.
Federer 6-1 6-6 (1-2) Youzhny*
Wow.
Youzhny comes into the net and hammers away a smash. He takes the first first mini-break.
Federer gets on the board a point later. The Russian challenged thinking the ball was wide but it did just clip the line.
*Federer 6-1 6-6 (0-1) Youzhny
Youzhny spanks the ball onto the baseline, Federer tries to pick it up, and lobs it wide. He challenges but that was very much in.