Federer leaves the court, but only temporarily, and in the meantime Monfils stretches the hamstrings. And breathe...

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"Out" yell a couple of spectators during a rally but the players carry on until Monfils nets on the opening point. "Please do not call out during the rally," says Carlos Ramons. The yips also affecting Federer - the Swiss chucking in a double fault for 30-15 - but a well-timed driving volley brings up two set points for the Swiss. Monfils hooks a return way beyond the tramlines. We carry on.

What do you do when 22,000 fans are screaming encouragement at your opponent? You go toe-to-toe with him. A cushion-soft touch at the net from Monfils levels matters. 30-30. Monfils seems to push a forehand into the tramlines. HawkEye proves it was a fraction wide. To deuce we go.

The pair trading blows from the baseline, screams from a section of the yee-hawingly vocal crowd as Federer drags a forehand wide. Advantage. Monfils attempts to pass Federer at the net but has no joy. Deuce. Double fault Monfils. His eighth of the match. Break point Federer. Monfils lethargic. Another double fault. There will be no sleeping on this watch.

While his forehand and first serve may have been off key in this match, Federer's net play has been on song. A volley at a zany angle. First point Federer but his forehand falters, gifting Monfils three easy points. Two match points Monfils! Oh my word.

The world watches on as a Monfils forehand bounces long, seemingly in slow motion. Federer comes to the net, the ploy pays dividends. Danger averted. Monfils goes long! Federer launches down and ace! The five-time champion jumps for joy! The crowd are on their feet! Federer lives on!

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A double fault from Monfils - his third of the set and seventh of the match - puts Federer on the scoreboard. 15-15. The Federer racquet sparkling - a forehand down the line for 15-30 - but from a high to a series of lows, a netted backhand, a forehand long, and Monfils nips to 40-30. The game is prolonged as the 20th seed screws a forehand long but, just as in previous games, the Frenchman pulls out two brutal serves from his magic box and Federer must serve to stay in the match.

Federer is three matches from making more history but Monfils is two games away from writing his own name into a chapter of US Open history. Will it be a first US Open semi-final for the Frenchman or a first semi-final since 2011 for Federer? Rapid-fire serves from Federer for a hold to 15 and the pressure quickly on Monfils once again.

Jill Craybas, US former professional tennis player on BBC Radio 5 live

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"It has been quite an intense match and the atmosphere has been awesome. The quality of tennis out here at this late stage of the match is incredible. We are seeing a bit more emotion from Federer, which is very unusual. I think he was surprised by how well Monfils came out at the start of this game, so he's really trying to pump himself to give himself some more energy.

Even with Monfils being two sets to one up, how close the statistics are show how tight this match is. This is the most focused I've seen Monfils in a match. There's usually more drama, shouting, jumping up and down."

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"Allez" screams Monfils as a Federer forehand whizzes past him and bounces long for 40-30, but super returning from Federer pushes the game to deuce. Monfils embarking on an entertaining monologue. Is he loosing it or is he revving himself up? It's the latter. An ace to the wings. An ace down the 'T'. Boom! Boom!

Monfils's two victories over Federer came on hard courts, but the Swiss does have the superior record. A hold to love from a stern-looking Federer. High-octane stuff.

A pumped-up Monfils cries an anguished groan after every shot, but it doesn't seem to be helping because Federer has reined him in to 30-30. A rush of blood to the head from Monfils and Federer has break point - but an overclubbed forehand into the tramlines from the Swiss lets Monfils off the hook.

The Frenchman talking to himself in his native tongue in between points. The heart is pumping. The adrenaline oozing. Two mammoth aces later - one a swashbuckling 137mph - and the exuberant Monfils is ahead for the first time in the set. Bravo!

The longer the rally, the greater chance of Federer coming out on top. Monfils stretching every sinew to fetch a Federer volley, but even a man of his elasticity struggles and he's outstretched on the floor, having allowed Federer to come back to 30-30. A third ace of the match from Federer but Monfils takes the game to deuce.

Break point Monfils, his second of the set, Federer saves. Thunderclap of a forehand for a second break point. Federer saves. Third break point - Federer volleys into the tramlines and whacks the net and decides to change his racquet. The Swiss exchanges words with the umpire. "What's wrong with you?" Federer asks Carlos Ramos.

Oh, Gael, what are you doing? A drop shot? non, non, non. Federer, like a shark smelling blood, senses that his opponent is feeling the strain. A dreamy backhand winner down the line from the former champion for two break points.

Monfils refusing to crumble, dragging the game to deuce - the forehand again Federer's foe - but the Swiss is volleying with aplomb and earns third break point. The Frenchman messes up on the baseline, forehand goes long, and Federer's stentorian roar can be heard in Staten Island.

Federer continuing to overcook the forehand and soon the alarm bells will ring. Some spectators shuffling uneasily in their seats as Monfils goes 0-30 up, but the Swiss digs deep, drags himself back into contention at 30-30 after coming out on top of a 21-shot rally.

"Come on," Federer roars as Monfils returns wide - but the second seed again wild on the forehand. Deuce. Jaw-droppingly good counter-punching from Monfils, a thunderous one-two, ending with a forehand into the corner for break point!

Monfils, the court at his mercy, slaps a backhand into the tape. Federer ups the ante - a fabulous deep backhand volley for advantage and another to see off the danger. Blimey.

Gigi Salmon
BBC Radio 5 live commentator

"This match is so level that both Federer and Monfils have hit 27 winners and unforced errors apiece.

"The difference in the third set was Federer made one unforced error compared to Monfils' nine."

Who said Monfils had questionable temperament? Cool and crisp serving to go 40-0 up and, other than a grand Federer forehand, the Frenchman has it all his own way.

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Monfils' radar a little off on the returns as Federer eases to 30-0. The Swiss scoots to the tape, a wonderful volley sends Monfils scrambling and the Swiss sees him off with an overhead. Three set points. He only needs the one. Simple, yet efficient from the Swiss master. We're in for the long haul.

In a blink of an eye, Monfils has rattled off three points quickly for a 40-0 lead and before I can add a full stop to the end of this sentence, he holds to 15. Federer to serve for the third set. Sleep? No, thanks.

Monfils continuing to retrieve what seems like the unreachable and his long limbs put Federer on the back foot in the first point, forcing the Swiss to net. Sharp-shooting from Federer, though, from 0-15 to 40-15, and he wraps the game up as Monfils ends a lengthy rally by swinging wide on the forehand.

There's a lovely ebb and flow to this match. Federer has momentum but the pendulum has swung with such regularity in this set you can never be sure who will pull through. A comfortable hold for Monfils, a challenge rightly going his way.

Gigi Salmon
BBC Radio 5 live commentator

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"Last time Federer came back from 0-2 down was against another Frenchman, Julien Benneteau, at Wimbledon in 2012. He's hade eight career comebacks from 0-2 down and needs to make it a ninth here to make it into the US Open semi-finals."