Excellent work from Youzhny to drag Federer forward while he stays camped on the baseline, and Federer's attempted volley ends up about 30 rows back.

Federer gets a brief lead as Youzhny's forehand goes long, but the world number three's backhand, so strong this year, isn't playing ball. Long, and we're at 30-30.

Oh! Youzhny could target Federer's backhand here but he chooses to go for the forehand - and he overworks it! Set point, Federer...

Youzhny had a slight limp before that last game. It's disappeared now. Amazing what adrenaline can do to a person.

Leon Smith
Great Britain Davis Cup captain on BBC Radio 5 live sports extra at Flushing Meadows

I am really surprised Roger decided to serve and volley there. Youzhny knew it was coming and it gave him a target.

Federer's done it again. Mirka, up in the stands, hides her face as Federer shanks a forehand long and hands Youzhny a break point.

He's broken back! A superb backhand, beating Federer who had advanced to the net, and Mikhail Youzhny has stopped Federer in his tracks once again.

Image source, Getty Images

15-0. Then 30-0. A loose forehand gets Youzhny on the board but then a backhand drop shot goes seven different kinds of wrong for Federer!

30-30.

Forehand winner after forehand winner for Federer to take him to 15-30.

Ooh, the backhand's still not ideal though, Federer's arm high in the air like a flamenco dancer as he nets to take it level, before another backhand puts Youzhny in front.

An ace out wide, and Youzhny forces Federer to serve for the set.

Here we go then. Youzhny will serve as Federer looks to force a decider.

Leon Smith
Great Britain Davis Cup captain on BBC Radio 5 live sports extra at Flushing Meadows

The good news for Roger Federer is how quickly he is getting through his service games. He is saving energy.

Image source, Getty Images

Another hold for Federer, the only fluff coming with an ill-advised lob.

Now, the last time he had a chance to take the set on Youzhny's serve, he fluffed his lines, and the Russian took the set on the tie-break. Will lightning strike twice?

Leon Smith
Great Britain Davis Cup captain on BBC Radio 5 live sports extra at Flushing Meadows

What a difference 15 minutes can make. Suddenly Roger Federer is in the driving seat.

He is doing alright. Whatever happens, if Roger was to win there are still big question marks.

A shank from Youzhny on the backhand sees the ball go sailing wide across the court.

And now a double fault - the first for a while from the Russian - but it's followed up with a fine backhand winner that Federer doesn't touch.

Youzhny is still pushing Federer into the corner of the court and the Swiss nets, before his aerial forehand drifts long. Youzhny holds.

Justin Stone: 5am start, I'll skip sleep for this match, 5 sets please Fed.

The physio/doctor/fella with a walkie-talkie has handed Youzhny one of those nasty-tasting energy gels.

He strides back on to court to serve.

Now then. Youzhny, not in the best of spirits right this second, has called for the physio.

You bet he can. A love hold for Fed.

This is superb stuff. Both players are using the full width of the court. Federer stretches to tap a return over before Youzhny's backhand finds the net.

Leon Smith
Great Britain Davis Cup captain on BBC Radio 5 live sports extra at Flushing Meadows

That has suddenly turned the tables. Now we are suddenly thinking Federer is OK - he can do this.

An almighty roar goes around the stadium as Federer gets a break point, dancing across the court, meeting Youzhny's aggression with two volleys and eventually the mistake came.

And he breaks! Youzhny's backhand goes into the net and Federer, for the first time in an age, breaks. Can he consolidate it with a hold?

Image source, AFP

Plenty of discussion about Federer's back. It hasn't, to my untrained eyes at least, looked to have been affecting his movement. He's just gone off the ball. Youzhny found his backhand at the time when Federer's went missing.

An ace, pushed out wide, is followed up by another loose backhand from Federer for 40-15. He wipes at his face with a towel and then comes up to the net, up on one leg to batter a forehand winner beyond his opponent.

That's a ridiculous shot from Federer! Down the line and Youzhny's response goes long.

This has been some turnaround from Youzhny, who looked all at sea in the first set. Not only had he not beaten Federer ever, he walked out knowing that 99% of that stadium was not on his side.

An ace and then a big serve from Federer hustle him to 40-15, and he holds as a Youzhny backhand drifts long.

Tasya: Fed not 100% fit - back suspect. Uncharacteristic errors.

NORZIELUS: Since the beginning of US Open 2017, Federer is moving in a very difficult way. By playing as such, he can't win the title.

Image source, Rex Features