The crowd, as you'd expect, rooting for the five-time champion. The 33-year-old continuing to make needless errors, but has enough tricks in his bag to consolidate the break - finishing off with a swashbuckling overhead which the fans thoroughly enjoy.

Canice: "Used to recall a time when Federer was almost invincible against this type of player..that seem like an era ago."

Greg Coles: "How well is Gael Monfils playing! Two sets up."

Usama Zafar: "At last a break of serve for Roger... If somehow he can take this game all the way towards a decider. NOT happening."

Monfils producing otherworldly shots on Ashe. A loose-limbed forehand touches the line for 30-30 - but from brilliant to bonkers as he coughs up a double fault to present Federer with a break point. And the Swiss matches his opponent for razzle-dazzle. Magnificent! He glides towards the tape, conjures a series of volleys until he knocks Monfils out. The second seed deserves to be allowed to put on his top hat and sing a show tune after that.

A frustrated roar from Federer as he watches his forehand drift wide. Monfils again with a chance at 0-30 and the second seed is creaking. Monfils pounces at the net after a lucky on a net cord and earns three break points.

One break point saved. Forehand winner. Second break point saved. Another forehand winner. How will it all end? Federer approaches the net... Monfils passes him with a stonking forehand which tickles the line. The second seed's advantage whistling in the wind.

Gigi Salmon
BBC Radio 5 live commentator

"The crowd inside the Arthur Ashe are stunned. They can't believe their beloved Roger Federer will have to come from two sets down to win. But now they are up and happy after he broke Monfils in the first game of the third set."

Monfils holding firm, refusing to cave in. Rat-a-tat-tat, take some of that. The Frenchman's serving in this game equally as good as Federer's in the previous one.

Federer serving like a man who has no doubts and no weaknesses. A hold to love and Monfils wipes his sweaty visage with a towel. The Frenchman knows he will have to battle to win this match.

Has Federer, even in his recent revived form, got enough in the tank to force this into a five-setter? At 30-30, Federer unleashes a fierce forehand and, unlike in the previous two sets, it hits the target, leaving Monfils swishing at fresh air. Break point Federer. Another vicious forehand and the comeback has begun. Hold on to your hats.

Two easy points for Monfils. 0-30. Nervous oohs from the New Yorkers. The Frenchman goes on to amass two set points, wastes one but Federer folds like a deckchair in the wind on the second - netting a forehand.

Trainers screeching... fans sighing... Federer misfiring. The Swiss wastes another opportunity to nick a point from Monfils, the forehand once again his undoing, but a rare error from Monfils and there's a chink of light for Federer at 30-30. But, poor old Roger, he soon realises that it's another dark tunnel ahead of him as Monfils serves with gusto. No chance.

Piers Newbery
BBC Sport at Flushing Meadows

Image source, Getty Images

"Monfils is totally confusing Roger Federer, he doesn't know if he's going to stay back or come into the net, and no theatrics either tonight," says Brad Gilbert on ESPN. Even the statistics are mildly eye-boggling, with Monfils winning just 59% of the points on his first serve but 77% behind his second serve. However he's doing it, he's getting it done.

A clumsy forehand into the tape from Federer sparks a collective groan from the spectators. At 30-15, Monfils is again knocking on the door but this time there's no way through as Federer's defence is as tough as teak. The five-time champion finishes off with a whipped forehand winner and the focus returns to the Monfils serve.

The second seed could soon be two sets down if Monfils continues to serve with such aplomb. He hasn't given Federer a sniff in this set - another hold to love and the Frenchman extends his advantage.

Image source, Getty Images

Federer the mechanic, tinkering under the bonnet, trying to find out why his engine is spluttering, and he comes up with a solution, a slight adjustment here and there and the first serve is firing on all cylinders again. His second ace of the match, mixed in with some unreturnable boomers, for a trouble-free hold.

The physio did eventually trot onto the court to examine Monfils, but the Frenchman's ankle doesn't seem to be troubling him. Federer unable to move the 20th seed around the court because the Frenchman fires four big first serves over the net, giving the Swiss little chance to start a fightback.

The trainer is lurking in the shadows as Federer unleashes one thudding serve after another for a comfortable love hold. The players return to their chairs, but the trainer remains standing on the sidelines. Perhaps he's not needed afterall.

Gigi Salmon
BBC Radio 5 live commentator

"Here's a stat to make Roger Federer fans cover their ears. In their nine previous meetings, whoever has won the first set has gone on to win the match."

Image source, Getty Images

Federer needs to dig deep and put previous miscues and mishits behind him. A slam dunk of an overhead takes Monfils to 30-15, but showmanship from Federer as he sprints to the tape and beats Monfils with a rapsing volley.

Federer slicing and dicing from the baseline but he comes unstuck, allowing Monfils to proceed to 40-30. But someone has flicked a switch and the second seed's engine is now roaring - an overhead, followed by an angry crosscourt volley and the Swiss has break point. Strange serving tactics from the Frenchman in that last point, but there's nothing wrong with his next howitzer, Federer sticks out an arm but can't return. Danger averted.

Monfils flat on his back and wincing as he touches his right ankle. The Frenchman twisting his ankle when slapping a forehand into the tape, but he gets back up on his feet and rattles off a quick point to hold.

Johnson Ogunniyi: "Gael Monfils has been like a box of chocolate in this year."

Femi Oyedola: "Sleepless in Lee, London, to watch Fed win but it's not looking great early on. He just might need a pep talk from the great Henry."

John McEnerney: "Fed to win in 4, needs to keep unforced errors count low & no lapses in concentration. Monfils is no pushover, a stern test is what Roger needs!"

John's on the ball as Monfils takes the first set - what's your prediction? Tweets to #bbctennis

Federer's first serve continuing to falter, just as it did in the first set, and his forehand isn't yet swinging freely, either. At 15-30 Monfils has a sniff, but the Federer forehand hauls the game to 30-30.

Monfils' forehand down the line wide by the length of a butterfly's wing. Oohs from the crowd as they watch the replay. Federer off the hook, for now at least.

The second seed's forehand faltering once more... deuce - advantage - deuce - Wow! Monfils returning the unreturnable but still he loses the point. But the Frenchman earns a break point, comes face-to-face with Federer at the net and outwits the 17-time Grand Slam champion, grabbing his chance thanks to a simple volley.