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
Right, that's us done for today.
We'll be back tomorrow for Kyle Edmund's match against Canada's Denis Shapovalov from 18:00 BST.
There'll be online only commentary available on 5 live sports extra, as well as the usual live text.
I'll leave you with a photo of Roger Federer looking bemused after another five-set marathon. In a bit!
Image source, AFP/Getty Images
Image source, Getty ImagesSo, what has gone on so far on day four, I hear you ask.
Well, the big shock of the day saw seventh seed Grigor Dimitrov knocked out by 19-year-old Russian Andrey Rublev.
World number one Karolina Pliskova almost suffered the same fate but came from behind to beat American Nicole Gibbs.
The first match on Arthur Ashe saw fourth seed Elina Svitolina progress while French Open champion Jelena Ostapenko is also into round three with her victory over Sorana Cirstea on Louis Armstrong.
Juan Martin del Potro's bid for a second US Open title continues as he won today but 15th seed Tomas Berdych is out. He lost to Alexandr Dolgopolov.
Another notable match saw American Shelby Rogers beat 25th seed Daria Gavrilova in the longest women's singles match in US Open history. It lasted three hours and 33 minutes.
Image source, Getty Images
Leon Smith
Great Britain Davis Cup captain on BBC Radio 5 live sports extra at Flushing Meadows
It is huge for the Federer team and huge for the tournament. He did a good job. He toughed it out. He is not playing well but is still in the tournament. He has still got motivation which is so important.
Federer stat alert...
The night session will get going shortly on Arthur Ashe Stadium.
Big-hitter Coco Vandeweghe will be on from 00:00 BST as she faces Tunisia's Ons Jabeur, before Rafael Nadal plays Taro Daniel.
American Madison Keys is still to come on Louis Armstrong Stadium against Tatyana Maria, while Svetlana Kuznetsova is last on Grandstannd Court against Kurumi Nara.
And Agnieszka Radwanska has just got her second-round match against Yulia Putintseva under way on court 17.
Image source, EPAAn update on that last entry - Thiem is through after winning the fourth set against Fritz 7-5.
It's all been about what was happening on Ashe for the past few hours but a quick update on what's going on elsewhere...
Sixth seed Dominic Thiem leads Taylor Fritz two sets to one and it's 6-5 to Thiem in the fourth on Louis Armstrong.
Gael Monfils and Donald Young are into a fifth on Grandstand - it's 3-3 in the decider.
David Goffin is two sets to one down against Guido Pella - it's 6-5 to Goffin on serve in the fourth.
Leon Smith
Great Britain Davis Cup captain on BBC Radio 5 live sports extra at Flushing Meadows
It makes it more difficult but he won three five setters in Melbourne to win the title. If he gets through the next matches quicker, which is more difficult, then he will be alright.
Federer might not give much away in the post-match interviews but it will be interesting. He doesn't play long five-set matches. This was just over three hours.
It's another 35-year-old next for Roger Federer - Feliciano Lopez, who overcame fellow Spaniard Fernando Verdasco in four sets today.
Federer's record against Lopez isn't quite as good as against Youzhny but it's still pretty formidable - 12-0.
But would you be confident putting any money on the Swiss after that display?
Image source, Getty ImagesRoger Federer speaking after his victory over Mikhail Youzhny: "The five-set battles are a lot of fun. I feel warmed up by now.
"I haven't played a lot of guys with cramps and the new rules have changed. You are not allowed to get support from the physio so it is hard to watch.
On Youzhny's cramp: "I feel for him. We go way back to the '98 juniors when we played doubles against each other. This was probably our best match so it was fun out here."
Leon Smith
Great Britain Davis Cup captain on BBC Radio 5 live sports extra at Flushing Meadows
It was amazing entertainment. It was not about the level. It was up and down from Roger and there was bravery from Youzhny who responded so well after the first set. Federer handed the initiative and Youzhny's confidence grew.
A total of 68 unforced errors for Federer, to go with 56 in his first-round match.
There's work to be done on the Federer backhand.
Only Jimmy Connors, with 98 victories, has won more US Open matches than Roger Federer.
It's some record.
Image source, ReutersFederer still looks fresh as he goes through the post-match routine. No questions about his back from the interviewer, though...
David Law
Radio 5 live sports extra commentator at Flushing Meadows
Federer hangs over the net exhausted. He might not be fully fit but he's still here.
Federer 6-1 6-7 (3-7) 4-6 6-4 6-2 Youzhny
Mikhail Youzhny departs to cheers from a stadium that seemed to be against him from the very beginning.
That's Federer's 80th win at Flushing Meadows.
Image source, Getty Images
Federer 6-1 6-7 (3-7) 4-6 6-4 6-2 Youzhny
He's done it!
An almighty match and Roger Federer raises his arms to the Arthur Ashe Stadium. He's through to the third round of the US Open after an engrossing three-hour battle.
Image source, Getty ImagesFederer 6-1 6-7 (3-7) 4-6 6-4 5-2 Youzhny*
Oh! Glorious from Federer, up on one leg and spanking the ball into the corner of the court. Can he do it on this one?
Federer 6-1 6-7 (3-7) 4-6 6-4 5-2 Youzhny*
That serve, though.
Federer can't pick it up and he backhands his return straight into the net. He goes long from out wide, though, and we're back to 40-40.