Roger Federer saved two match points to defeat Gael Monfils 4-6 3-6 6-4 7-5 6-2 and reach his ninth US Open semifinal, keeping alive his dream of a sixth title.
The 33-year-old Swiss second seed, bidding to become the oldest winner of a Grand Slam title in more than 40 years, was staring down the barrel of a demoralising exit when French 20th seed Monfils had two match points in the 10th game of the fourth set.
But once they were saved, the lifeblood was suddenly sucked out of the flamboyant but unpredictable 28-year-old Frenchman, whose notoriously fickle stamina failed him in a one-sided fifth set.
Victory was Federer's 26th in 27 night-time matches in New York and put him just one win away from a career 600 victories on hard courts, a landmark he can reach if he defeats Marin Cilic to make the final. Croatian 14th seed Cilic toppled sixth-seeded Czech Tomas Berdych 6-2 6-4 7-6 (7-4) earlier today to reach his first US Open semifinal.
Federer's win was his ninth from two sets to love down and first at a major since he had to engineer a similar Houdini act against another Frenchman, Julien Benneteau, at Wimbledon in 2012 on his way to his 17th and most recent major title.
"Gael played great tennis but even when I was two sets down I knew the finish line was still far away for him," said Federer, in his first semifinal since 2011. "I knew I could play better tennis but on match point I wasn't feeling so great anymore.
"I just thought, 'this could be the last point, don't give it away on an easy shot, make him work for it'."
Federer was his usual composed self at the start with a service game to love but it was Monfils who carved out the first break for a 3-2 lead off a skewed Swiss forehand. The set was the Frenchman's as Federer pushed a tame backhand off a second serve wide.
Monfils broke again for a 1-0 lead in the second as Federer continued to struggle with the humid, windy conditions as well as his own shortcomings. Uncharacteristically, the 17-time major winner lost his famed self-control in a testy exchange with umpire Carlos Ramos as he changed ends.
Monfils then added to the drama by rolling his right ankle, collapsing the the ground and calling the trainer but still going on to hold serve for 3-1. He then backed up the break by taking the set 6-3 as another Federer backhand crumbled into the net.
Two sets had taken just 78 minutes with Federer unable to fashion a break. However, that changed as he grabbed his first break of the tie in the opening game of the third set, before it was retrieved in the fourth.
Federer, though, hit back immediately for 3-2 and claimed the set in the 10th game, having committed just one unforced error compared to 26 in the first two. It was also the first set that Monfils, who put out seventh seed Grigor Dimitrov in the last round, had dropped in the tournament.
A thrilling, contentious fourth set saw breaks exchanged to 2-2 with Federer smashing his racquet into the net and berating Ramos: "I don't know what's wrong with you".
In a tense set, Federer saved two match points in the 10th game before Monfils served up two successive double faults to hand his rival a lifeline of a 6-5 lead and a straightforward service game for 7-5 and two sets apiece.
From there it was business as usual with a dispirited Monfils broken in the first and fifth games as Federer moved into his 36th Grand Slam semifinal.